Effect of Impurities on properties of alumina

There are 15 kinds of impurity elements affecting the properties of alumina, and 5 kinds of impurity elements are common and especially harmful. They are Na2O, Fe2O3, SiO2, CaO, MgO and so on. The following is a brief analysis of some impurity elements and their effects on the properties of alumina.

The main results are as follows: 
1. The influence of impurity Na2O on the dielectric loss of alumina sintered ceramics is significant.
The increase of sodium oxide content is usually accompanied by the significant increase of tg δ (tg δ is the tangent of the dielectric loss angle). The harmful effect of sodium oxide on tg δ improvement of alumina porcelain is related to the presence of Na2O ·11Al2O3 is formed by the reaction of Na2O with Al2O3 at high temperature (above 1400 ℃), also known as β-Al2O3.
In addition, in the forging process, sodium aluminate will also affect the conversion rate and conversion of Al2O3, and make the grain size of alumina coarser, the specific surface area reduced, and the crystal shape irregular. Its existence also damages the electrical properties of ceramics and reduces the degree of fire resistance. Therefore, Na2O is a harmful impurity and should be removed as far as possible.
2. The influence of impurity Fe2O3 
Mainly in the following aspects: the sintering temperature of ceramics becomes narrower; the cold and hot properties of ceramics are reduced; the electric breakdown strength and flexural strength of ceramics are reduced; Influence the whiteness of ceramics, but also easy to produce spots on ceramic glazes. Although the content of iron oxide in raw materials is relatively low, the secondary pollution caused by wear and tear of iron equipment in the process of forging and sintering causes the iron content to exceed the standard. Therefore, magnetic iron removal must be carried out in the production process to avoid and eliminate the secondary pollution that may occur in the production process.
3. Influence of impurity SiO2
SiO2 in ceramics will exist in the secondary crystal phase of mullite (3Al2O3·2SiO2). If SiO2 increases and the secondary crystal phase increases, the content of main crystal phase -al2o3 will decrease, which will affect the performance of ceramics. The content of SiO2 in low-sodium alumina in China is 2-3 times higher than that in foreign countries , not because the raw materials in China contain too much silicon. The main reason for the increase of impurity SiO2 is that some factories have poor sanitation conditions, lax production management and weak product quality awareness, which results in the mixing of dust and kiln refractory brick powder, causing secondary pollution. 
4. Influence of impurities CaO and MgO
CaO and MgO formed calcium CaO·Al2O3 and spinel MgO·Al2O3 secondary crystal phases in ceramics. If CaO and MgO content increased, the main crystal phase -al2o3 decreased. Therefore, western countries and China's military alumina regulations: the sum of Cao and Mgo content cannot exceed 0.05%.