Black alumina ceramics

As a non-consumer industrial application material, people usually emphasize the mechanical and electrical properties, chemical stability, heat resistance and other properties of special ceramics, and pay little attention to its color. It's just like with a man of science and technology, we focus on his knowledge and professional level, but it doesn't matter whether he is handsome or not. But there are always exceptions, such as black alumina ceramics is a value of "appearance level" material. But in the end, "black" is not usually a beautiful color, black alumina ceramics in recent years more and more attention, not because of its "appearance level", or because of its special practicability - can block light, black can absorb all wavelengths of visible light, not let the light through.

What kind of applications need shading? Mainly semiconductor integrated circuits. Semiconductor integrated circuit has obvious photosensitivity, so it requires the encapsulation of the shell to have the performance of shading. So why does it have to be alumina? Because alumina has excellent comprehensive properties, such as high resistivity, high thermal conductivity, very low dielectric loss, etc., of course, but also the price is relatively low, can be mass-produced - in short, cost-effective.

We all know that alumina ceramics are originally white, how to make it black? The surface brush black paint is the most direct, but in that case, thermal conductivity, heat resistance and anti-aging properties will be greatly reduced, affecting its use effect. Therefore, it is generally mixed with black colorants, so that the alumina itself becomes black. These colorants are usually transition element oxides, such as Fe2O3, CoO, NiO, Cr2O3, MnO, TiO2, V2O5, etc. How to add it? If it's simpler, just use a colorant. For example, some people in the ordinary 95 alumina formula to add 3%TiO2, into the hydrogen molybdenum wire furnace, sintered at 1650-1680℃, to obtain black alumina ceramics, used as a digital tube liner. However, there are often many shortcomings in doing so, such as high sintering temperature, high energy consumption, and easy to cause volatilization of transition oxides at high temperatures, and so on. Therefore, the more reliable approach is to select several colorants at the same time to form a low eutectic, which can reduce the sintering temperature. For example, there are studies that add 3-4% manganese oxide and titanium oxide to 99 alumina, and the sintering density can reach 3.75g/cm3 at a low temperature of 1250℃.

The specific adding methods of colorant are divided into primary synthesis method and secondary synthesis method. The so-called one time synthesis method is the coloring oxide and other additives together, directly added to the alumina powder mixed, formed after a one-time firing. However, the secondary synthesis law is that the secondary synthesis method is to first mix the metal oxide calcined to synthesize the black color; Then the obtained color, co-solvent and alumina are prepared according to a certain ratio and process. In contrast, the primary synthesis method is relatively easy, but the secondary synthesis method is better. Why is that? Because when metal oxides are mixed and calcined, spinel structure compounds are formed. Compared with oxides, these compounds are not easily volatile at high temperatures, so the proportion of components in alumina is more accurate, and the black obtained is more pure and stable - of course, this is not to say that a synthesis method will not form spinel structure compounds, but the number and uniformity will be worse, so the coloring effect is also worse.

Since colorants are metal ions of variable value, the mechanical and electrical properties of alumina will inevitably decline. For example, in the previous example of 95 alumina formula adding 3%TiO2, the alumina is black mainly because some Ti4+ ions become valence Ti3+. Because the extra electrons on the trivalent titanium are weakly bound electrons, it is easy to become conductive carriers, so that the resistivity of the ceramic is reduced to less than 108Ω·cm. In order to avoid excessive reduction of resistivity, when preparing black alumina ceramics, it is often necessary to add some adjustment additives. For example, when adding TiO2, the introduction of MgO can inhibit the transformation of Ti4+ ions to Ti3+ ions. For example, some people in the preparation of black alumina ceramics, adding 4%SiO2, the result is that the resistivity is an order of magnitude higher than before the addition, while the dielectric loss is an order of magnitude lower than before the addition.

Is it true that black alumina ceramics can only be used for shading occasions? Of course not, in fact, if the color is not too particular, many places that use white alumina ceramics can use black alumina ceramics, and because the sintering temperature of black alumina ceramics is usually relatively low, there may be some advantages on the cost - of course, there is a premise here, It is to adjust the performance indicators of black alumina ceramics to the same as that of ordinary white alumina ceramics.

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