As mentioned earlier, one of the necessary conditions for achieving ceramic sintering is that there should be mass transfer during the sintering process. Under the action of the sintering driving force, only through the material transfer process can the pores be gradually filled, so that the green body becomes dense from loose. At present, the research on the sintering mechanism, that is, the research on the mode and mechanism of material transfer in the sintering process, has four main theories, namely evaporation and condensation, diffusion, viscous flow and plastic flow, dissolution and precipitation. It should be pointed out that the material transfer phenomenon in the actual sintering process is quite complicated, and it is impossible to use a certain mechanism to explain all the sintering phenomena. Therefore, most scholars believe that there may be several mass transfer mechanisms in the sintering process.However, under certain conditions, a certain mechanism dominates, and when conditions change, it will play a dominant role.
The mechanism used may also change accordingly.
(l) Vaporation and condensation On a curved surface, such as any part of spherical particles (spherical crown), the neck between two particles, pores in ceramic blanks, etc., a curved surface pressure will be generated under the action of surface tension p, let the radius of curvature of spherical particles be r and the surface tension be d. Then we get: p=2a/r It can be seen from the above formula that the smaller the curvature radius, the greater the surface pressure p. when r is close to . . When the surface is a plane, p=0; for a convex surface, p>0, which means that the vapor pressure on the surface is higher than that of a plane; for a concave surface, p<0. Indicates that the vapor pressure on the surface is less than the plane vapor pressure.
Particles with curved surfaces have excess surface free energy △Z:△Z—Vp—2aV/r compared to flat surfaces where V is the molar volume. It can be seen from the above formula that △Z>0 for the convex surface particles, △Z=0 for the plane surface, and AZ It can be seen from the above analysis that due to the difference of vapor pressure on different curved surfaces, there is a gas-phase transfer process of matter evaporation on the convex curved surface and substance condensation on the concave curved surface. Figure 4-29 shows the two-sphere model of this transfer process.
The ceramic powder transfers substances through evaporation and condensation. As a result, the contact area between the particles increases. Therefore, the kinetic information of the transfer mode of the substance can be obtained by calculating the change of the contact area of the particles. The contact area between the particles The increase of α is faster in the initial stage of material transfer, and gradually weakens with the progress of sintering time. This is mainly due to the small radius of curvature of the concave surface between the particles at the initial stage of sintering. As the sintering progresses, the concave curved surface gradually becomes flat, and the sintering driving force generated by the vapor pressure difference gradually decreases.
Figure 4-31 is a Nivi photo of sintered sodium chloride, which clearly shows the change in the contact area between particles: at the initial stage of sintering, the particles are mainly in point contact, and after 90min of sintering, the sodium chloride particles become obviously flat , the contact area between particles increases. It should be pointed out that the shape of the pores in the ceramic body is changed by evaporation-agglomeration to transfer the material on the particle surface to the neck of the particle, but the particle spacing does not necessarily decrease. This is also well illustrated. This means that the sintering process of gas transport only changes the shape of the pores and affects the performance of the product, but the porosity of the product does not change, and the product does not densify.
It can be seen from the previous analysis that the gas phase transport is closely related to the initial radius and temperature of the particle: the initial radius of the particle determines the radius of curvature of the neck when gas phase transport occurs, and the temperature determines the vapor pressure. Therefore, for the selected raw ceramic powder (determined by the particle radius), it must be heated to a certain temperature to bring the vapor pressure to a usable level. For micron-sized ceramic particles, this vapor pressure is between 106 and 10-5 MPa (10-5 – 10-4 atm), such as the gas phase transport that occurs during sintering of sodium chloride. However, the vapor pressure required for gas phase transport of ceramic materials such as compounds is much higher than the above-mentioned values, and thus the required temperature is also high. In this case, other modes of mass transport have often dominated.
(2) Diffusion The ceramic raw material with low volatility at high temperature is mainly transferred through surface diffusion and volume diffusion. At this time, sintering is realized by diffusion. There are often many defects in the actual crystal. When the defect has a concentration gradient, it will diffuse directionally from the place with high concentration to the place with small concentration. If the defect is an interstitial ion, the diffusion direction of the ion is the same as that of the defect; if the defect is a vacancy, the diffusion direction of the ion is opposite to that of the defect. The more vacancies in the crystal, the easier it is for ions to migrate. The diffusion of ions and the diffusion of vacancies are both material transfer processes. The sintering caused by diffusion can generally be described by the concept of vacancy diffusion. There are many factors that affect diffusion mass transfer, such as chemical composition, particle size, temperature, atmosphere, microstructure, lattice defects, etc. of the material, among which temperature and composition are the most important. In ceramic materials the diffusion coefficients of both anions and cations must be taken into account, generally the slower diffusing ions control the overall sintering rate. Adding sintering additives, increasing the number of vacancies, also affects the sintering rate due to changes in the diffusion rate.
(3) Viscous flow and plastic flow liquid phase sintering refers to the sintering in which the liquid phase exists due to the temperature higher than the melting point of a certain component during the sintering process. The basic principle is similar to that of solid phase sintering. The driving force is still surface energy. The difference is that the sintering process is closely related to the amount of liquid phase, the properties of the liquid phase, the solubility of the solid phase in the liquid phase, and the wetting behavior. Therefore, liquid phase sintering kinetic studies are more complicated than solid phase sintering.
- ①Viscous flow When the content of liquid phase is high, the liquid phase has the flow properties of Newtonian liquid, and the sintering of this powder is easier to achieve equilibrium through viscous flow. In addition to the viscous flow of sintering in the presence of liquid phase, crystalline particles are also considered to have flow properties at high temperature, which is the same as the viscous flow mechanism of amorphous at high temperature. The viscous flow of matter at high temperature can be divided into two stages: in the first stage, the matter forms a viscous liquid at high temperature, and the centers of adjacent particles meet each other, increasing the contact area, and then the cohesion between particles occurs and the formation of some In the second stage, the viscous compaction of the closed pores, that is, the small pores are compacted due to viscous flow under the action of the surrounding pressure of the glass phase. There are three main parameters that determine the sintering densification rate: initial particle size, viscosity, and surface tension. The two main parameters, the initial particle size of the raw material and the viscosity of the liquid phase, cooperate with each other. They do not act in isolation, but affect each other. In order to make the liquid phase and solid phase particles combine better, the viscosity of the liquid phase should not be too high. If it is too high, additives can be added to reduce the viscosity and improve the wetting ability between the solid and the liquid phase. However, the viscosity should not be too low, so as to avoid gravity flow deformation due to excessive gravity when the particle diameter is large. That is, the particles should be confined within an appropriate range so that the effect of surface tension is greater than that of gravity. Therefore, in liquid phase sintering, fine-grained raw materials must be used and the particle size of the raw materials must be reasonably distributed.
- ②Plastic flow reduces the amount of liquid phase combined in the green body at high temperature, while the solid phase content increases. At this time, the sintering mass transfer cannot be regarded as a Newtonian fluid, but a fluid belonging to plastic flow, and the driving force of the process is still the surface energy. . In order to achieve dense sintering as much as possible, the smallest possible particle size, viscosity and large surface energy should be selected. In the solid-liquid two-phase system, when the amount of liquid phase is the majority and the liquid phase scrap is low, the sintering mass transfer is dominated by viscous flow, while when the amount of solid phase is in the majority or the viscosity is high, the plastic flow force is dominant. In actual sintering, in addition to different solid phases and liquid phases, there are pores, so it is much more complicated.
The plastic flow and mass transfer process also exists in pure solid phase sintering. It can be considered that the flow of crystals under the action of high temperature and high pressure is due to the slip of crystal planes, that is, the generation of dislocations between lattices, and this slip is only It only begins to occur when a certain critical stress is exceeded.
(4) Dissolution-precipitation The following mass transfer process occurs between the solid and liquid phases during sintering: the solid phase is dispersed in the liquid phase, and is rearranged in the neck through the capillary action of the liquid phase to form a tighter accumulation. Fine particles (which have higher solubility) and generally the raised surface portions of particles dissolve into the liquid phase, and settle out by transferring the liquid phase to the surface of coarse particles (where solubility is lower). This mass transfer process occurs in a material system with the following conditions: sufficient liquid phase is formed; the liquid phase can wet the solid phase; the solid phase has proper solubility in the liquid phase. The driving force of the dissolution-precipitation mass transfer process is the capillary pressure of the liquid phase between the fine particles, and the mass transfer process proceeds in the following manner. First, as the sintering temperature increases, a sufficient amount of liquid phase appears. The solid-phase particles are dispersed in the liquid phase. Under the action of the liquid-phase capillary, the particles move relatively and rearrange to obtain a tighter packing, resulting in an increase in the density of the green body. The ratio of the amount of shrinkage to total shrinkage at this stage depends on the amount of liquid phase. When the volume fraction of the liquid phase is greater than 35%, this stage is the main stage to complete the shrinkage of the green body, and its shrinkage rate is equivalent to about 60% of the total shrinkage rate. Second, bridging between particles separated by a thin liquid film leads to plastic deformation and creep due to high local stress at the contact site. This promotes further rearrangement of the particles. Third, through the recrystallization process of the liquid phase, this stage is characterized by the dissolution of the fine particles and the raised parts on the surface of the solid particles, transferred through the liquid phase and precipitated on the surface of the coarse particles. The green body is further densified while the particles grow and change in shape. When there is a liquid phase between the particles, the particles are pressed against each other, and the solubility of the solid substance in the liquid phase is improved under the action of the pressure between the particles.
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