Material’s parameter particle size influences sintering
What material’s parameter influences sintering? In the sintering process of specific ceramic products, material-related parameters significantly impact powder densification and grain growth.
Particle size
The smaller the particle size in the original powder, the faster the densification rate. Given that the particles have the same characteristics, under certain temperature conditions, the sintering time required for a row of spherical particles with a radius of r1 is t1. In contrast, the sintering time for another row of identically arranged spherical particles with a radius of r2 is t2, then:
T2=(r2/r1)nt1
In the formula, the size of n is related to the mass transfer mechanism during sintering. For lattice diffusion and grain boundary diffusion, n is generally 3~4. When we reduce the particle size from 1μm to 0.01μm, we can reduce the sintering time by 106 to 108 orders of magnitude. At the same time, small particle sizes can increase the density of the sintered body, reduce the sintering temperature, and reduce the sintering time.
However, reducing particle size can lead to issues such as caking and agglomeration. Extremely fine particles are more likely to adsorb significant amounts of gases or ions (e.g., CO₃²⁻, NO₃⁻, Cl⁻, OH⁻). We must remove these adsorbates at very high temperatures, which can impede particle contact and hinder the sintering process. Additionally, very fine particles are susceptible to secondary recrystallization, where they dissolve and redeposit onto larger particles, affecting the final material properties.
We must select the particle size reasonably according to the sintering conditions. The appropriate starting sintering particle size for materials such as Al2O3, MgO, UO2, and BeO is 0.05~0.5μm.
powder caking and agglomeration
Agglomeration occurs when a small mass of particles is held together by surface forces and/or solid bridging. This process results in particles being firmly bonded and/or chemically reacted to form a larger particle. The gaps between agglomerates and agglomerates are larger than the gaps between the constituent particles, and larger gaps require longer sintering times. In addition, the densification of the internal particles of each agglomerate or agglomerate will lead to shrinkage, which will further increase the gaps between them. Agglomeration and agglomeration can occur at several stages of powder preparation, as shown in Figure 1.
During ball milling
“The liquid medium causes van der Waals attraction between particles, which binds them together to form agglomerates. Correspondingly, powders will also produce a diffusion charge layer in the adjacent liquid medium. The overlap of two particle diffusion layers will produce a repulsive force, which can prevent the particles from approaching each other.
Studies have found that the dispersion performance and degree of dispersion of many oxide powders in aqueous solution are controlled by the pH value of the aqueous solution. Therefore, improve the agglomeration of powders by adjusting the pH value. It also uses polymer solutions to stabilize colloidal dispersions. When particles wrapped in organic matter approach each other, the overlap of adsorbed polymers will produce repulsive energy and repulsive force. This repulsive force is mainly caused by the increase
During drying
Residual moisture creates liquid bridges between the necks of the particles. The capillary pressure within the liquid bridge creates an attractive force between the two particles, comparable to the van der Waals force. The crystallization of salt can produce solid phase bridging. The strength of the salt bridge depends on the strength of the bridge and the strength of the bond between the solid phase particles and the crystallized salt. The concentration of the salt solution in the powder slurry determines the average neck size of the “salt bridge”. Precipitating the salt by rinsing (washing) or chemical treatment can eliminate the salt bridge phenomenon and prevent agglomeration and agglomeration of particles.
During calcination
The solid phase bridges formed are mainly due to partial sintering or neck growth between solid phase particles. If loose agglomerates have been formed during the particle preparation process, the heat treatment during the calcination process will transform these agglomerates into harder agglomerates. Since the size of the sintered neck increases with the increase in calcination temperature, the bonding strength of the agglomerates increases with the increase in temperature. Broke up these agglomerates usually by mechanical energy through ball milling.
Material’s parameter particle shape influences sintering
The shape of the particles has a certain influence on the sintering performance.
During liquid phase sintering, the particle shape has a greater impact on the densification rate and the volume fraction of the liquid phase. There is a difference between the capillary force between wetting spherical particles and the capillary force between irregularly shaped particles. Figure 2 shows the curve of the force between particles of different shapes as the volume content of the liquid phase changes.
When a small volume of liquid phase into the contact area of
Particle size distribution
The effect of particle size distribution on the final density of the sintered sample can be studied by analyzing the relevant kinetic processes. That is, the balance between the “porosity removal” and the driving force of grain growth during sintering is analyzed for particles of different sizes and distribution in the green body. The grain size distribution in the sintered sample is similar to the starting particle size distribution. The results show that a small particle size distribution range is necessary to obtain a high sintered density.
Material’s parameter admixture influences sintering
When the ion size, lattice type, and valence number of the additive and the sintering phase are close, they can form a solid solution. The main crystal phase lattice is distorted, and defects increase, which is conducive to diffusion and mass transfer, thereby promoting densification. The formation of a limited solid solution can promote sintering more than the formation of a continuous solid solution. The greater the difference between the ion valence and radius of the additive and the sintering phase, the greater the degree of lattice distortion, and the more significant the effect of promoting sintering.
The compound formed by the additive and the sintering phase is conducive to inhibiting the grain boundary movement speed and promoting densification.
The additives and certain components of the sintered body form a liquid phase, in which the diffusion and mass transfer resistance are small and the flow and mass transfer speed are fast, which reduces the sintering temperature and improves the degree of densification.
However, the amount of additives must be appropriate, otherwise it will hinder sintering. Too many additives will hinder the contact between sintering phase particles and affect the mass transfer process.
Back to SINTERING TECHNOLOGY
You might be interested in:
