Sintering aid is a kind of material that can promote materials to be sintered to near theoretical density at a temperature much lower than the melting point of the material itself. However, the mechanism of this kind of material to promote sintering is very complex. The sintering mechanism of the same material with different sintering aids may be different. The mechanism of sintering aids to promote sintering is as follows:
(1) It forms solid solution with sinter. When the solid solution is formed between the sintering aid and the sinter, the crystal will be distorted and activated, which can reduce the sintering temperature and increase the diffusion and sintering speed, which is particularly important for the formation of vacancy type or interstitial type solid solutions. If Cr2O3 is added during Al2O3 sintering, continuous solid solution can be formed due to the close positive ion radius of Al2O3 and Cr2O3. When TiO2 is added, the sintering temperature is lower. Because the size of Ti4+ions is the same as that of Cr3+ions, in addition to being solid soluble with Al2O3, and because the electricity prices of Ti4+and Al3+are different, positive ion vacancies will occur after replacement, which can effectively promote sintering.
(2) Prevent crystal transformation. Some oxides will undergo crystal transformation with large volume effect during sintering, which will make sintering densification difficult and easily lead to green body cracking. Selecting appropriate sintering aids can inhibit the volume effect and promote sintering, such as adding a certain amount of CaO to the ZrO2 sintering opening. At about 1200 ℃, the stable monoclinic ZrO2 is transformed into square ZrO2, which will be accompanied by 10% volume shrinkage, thus deteriorating the stability of the product. If Ca2+ions with lower electricity price than ZrO4+are introduced, cubic Zr1-xCaxO2 stable solid solution can be formed. This not only prevents the product from cracking, but also increases the vacancy concentration in the crystal, which accelerates the sintering.
(3) Inhibit the abnormal growth of granules. In general, grain growth is beneficial to sintering at the later stage of sintering. However, if secondary recrystallization or intermittent grain growth is too fast, reverse densification will occur due to grain coarsening and boundary widening. At this time, the densification process can be promoted by adding sintering aids that can inhibit grain growth. For example, adding MgO into Al2O3 is a good example. However, it should be pointed out that normal grain growth is beneficial, and the abnormal grain growth needs to be eliminated and controlled.
(4) Produce a suitable liquid phase. If there is a suitable liquid phase during sintering, it will greatly promote the particle rearrangement and mass transfer process, or the liquid-solid reaction will generate a new cement phase at a lower temperature to promote sintering. The appearance of liquid phase may be due to the low melting point of the sintering aid itself, or it may form a low eutectic with the sinter. Adding boric acid or boric anhydride to the dry vibrating material is such a type.
The fired products of refractory bricks produced by Kerui Refractory include clay bricks, high alumina bricks, light insulating bricks, silica bricks, magnesia bricks, carbon bricks, and composite refractory products, carbonated oxide refractory products, etc.
