Interesting talk about "alumina ceramics"

Yes, the non-shiny stuff above is essentially alumina, but today I won't talk about how to identify gems. Today, I'll take you to understand the application of alumina in industry.

Al2O3 mainly exists in the form of bauxite (Al2O3 · 3H2O) in nature, and the mainstream process is formed by Bayer method. The process is probably as follows:

Al2O3 · 3H2O + NaOH → NaAlO2 (solution), the slag is discharged after filtration,

NaAlO2 + Al (OH) 3 → Al (OH) 3 (cooling supersaturated deposition),

Al (OH) 3 → Al2O3 (baked at 900~ 1200 ℃) form α-type crystals,

Al (OH) 3 → Al2O3 (dehydrated at 140~ 450 ℃), forming γ-type crystals.

The specific process flow is as follows:

The essence of this red process is to powder the ore and remove the impurities in the bauxite by sufficient chemical means to achieve the purpose of purifying alumina. Our regular 95%, 99%, 99. 5% is basically determined by the process of blue font. Don't underestimate the decimal places, this already belongs to the category of fine chemicals, and it is also a symbol of the strength of basic technology. Is there a feeling of an ugly duckling turning into a white swan? However, human beings are not satisfied with obtaining a white swan, but make swans into various sumptuous delicacies (let'sdoit). Before making delicacies, it is necessary for us to understand the types of white swans. Let's take a look at the main crystal forms of alumina.

There are about a dozen crystal forms of alumina in general. But the three most commonly used/seen are: α-crystal form, γ-crystal form, and β-crystal form. Among them, α-type is used the most (meaning that the unit price is probably not high).

Crystal transformation relationship of different alumina ceramics and 6-square crystal form α-Al2O3

Here is a small episode. Beta-Al2O3 was first discovered in the refractory material (Al2O3) on the cellar fireplace where glass is produced. At first it was thought to be a new Al2O3 crystal form, but it turned out to be a multi-compound formed by a part of Na2O penetrating into the Al2O3 (so strictly speaking, this should be a composite material sigma (⊙▽⊙ "a), hahaha. But this special conductivity immediately made it ranked among the" functional materials ", and its value immediately soared.

Beta-Al2O3 batteries are more efficient than traditional batteries

Take this small series to tell you a truth of life: in ordinary positions, the more practical and diligent, can withstand tempering, and actively absorb environmental nutrients, quantitative changes will always cause qualitative changes one day (cheap refractory bricks absorb Na seconds to change batteries, worth skyrocketing).

A strong sentence needs to be inserted here: The characteristics of a material are determined by its composition and structure. The production process affects the composition and structure. For example: Al2O3 obtained from bauxite by sedimentation method is often flocculent; further drying and dehydration agglomerate to form a mixed phase of γ crystal phase and α crystal phase, which looks like a mass of powder. In fact, there are many van der Waals forces at the microscopic level, which is a loose material with a structure. When the temperature rises to 1200 ° C, some Al-O chemical bonds will be formed, and γ-crystal clusters will further fuse to form more dense α-crystal clusters. The loose structure means a large surface area, so the γ-crystal phase has an awesome feature that the specific surface area is particularly large, up to 300m2/g (the bamboo charcoal we commonly use is 350m2/g), so it is basically a chemical catalyst, a star product in the gas adsorption industry. Corrosion resistance and high temperature resistance are enough to kill other catalyst catalysts in seconds. But it is a waste to use the coating as a protective film. What should I do if I want to enjoy the high-strength protection of the alumina strip? I can only choose denser α-phase alumina ceramics. However, both are white in appearance and must be used with caution.

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