Let's understand the concept of "alumina". According to the international general classification standards, it is usually divided into two categories: metallurgical grade alumina and non-metallurgical grade alumina. Non-metallurgical grade alumina is also called multi-variety alumina and chemical alumina. Among them, pseudo-boehmite, boehmite, ultrafine aluminum hydroxide, activated alumina, calcined alumina and corundum all belong to the category of chemical alumina.
What we are going to talk about today is the relationship between this chemical substance with the chemical formula "Al2O3" and pseudo-boehmite and boehmite.
一, Differences between composition and application
1. Al2O3
Currently, there are more than a dozen crystal forms of Al2O3, such as χ, ρ, γ, κ, δ, θ and α, among which α and γ are the two most widely used.
α- Al2O3 has a Mohs hardness of about 9, a melting point of 2050℃, a boiling point of 2980℃, and a thermal conductivity of 28.88W/(m·K). It has good chemical stability, high temperature resistance and wear resistance, and can be used as a basic raw material for refractory materials, ceramics, abrasives, etc. Taking advantage of its high thermal conductivity and good insulation, low-sodium calcined alumina is used as a thermal conductive filler in the electronics, information, new energy and other industries. Low-sodium alumina also has the characteristics of good electrical insulation, high refractoriness, and small sintering shrinkage. It is a high-quality raw material for manufacturing electronic substrates, spark plugs, vacuum tube shells and other electrical components, as well as heat-resistant or wear-resistant ceramics. In addition, low-sodium ultrafine alumina has been widely used in the production of diaphragm materials for lithium batteries in recent years.
Crystal structure of α-Al2O3
γ-Al2O3 has the characteristics of large specific surface area, good adsorption performance and good surface acidity. It is a widely used activated alumina. It can be used as a drying adsorbent, fine polishing abrasive, catalyst and catalyst carrier. It is used in petrochemical, gas drying, automobile exhaust and industrial organic waste gas purification and treatment and other fields.
γ-Al2O3 crystal structure
2. Boehmite
Boehmite (also known as monohydrate soft aluminum stone or boehmite, chemical formula γ-AlOOH) is an active alumina precursor with high crystallinity and stable structure. Compared with pseudo-boehmite, its molecular arrangement is more orderly, so it has better thermal stability and chemical stability.
Boehmite can withstand high processing temperatures, so boehmite can be used as a heat-resistant flame retardant and applied to halogen-free copper clad laminates, insulation products, device plastic shells, etc. that need to withstand high processing temperatures. In addition, boehmite is the main raw material for the production of active alumina catalysts for coal chemical industry, etc. It can also be used as a raw material for the production of lithium battery diaphragms, β-Al2O3 for sodium-sulfur batteries, and a precursor for calcined alumina. Low-sodium ultrafine alumina prepared with low-sodium ultrafine boehmite as a precursor has the characteristics of low sodium, high conversion rate, high insulation, and good sintering performance. It has been widely used in lithium battery diaphragms, microcrystalline ceramics, precision ceramics, foam ceramics, integrated circuit chips, aviation light source devices, etc.
3. Pseudo-boehmite
Pseudo-boehmite is an incompletely crystallized boehmite with a similar chemical composition to pseudo-boehmite. It contains 1.25 to 2.0 crystal waters and is a type of incompletely crystallized hydrated aluminum oxide, also known as monohydrated aluminum oxide and pseudo-monohydrate soft aluminum oxide.
The typical structure of pseudo-boehmite is an extremely thin folded sheet. It is an aluminum oxide hydrate with a special spatial network structure. It has physical properties such as high specific surface area and large pore volume and chemical properties of peptization and thixotropy under acidic conditions. These properties of pseudo-boehmite make it widely used in many industries. It is used as a catalyst, carrier, and binder in the petroleum industry; as a carrier coating for automobile exhaust treatment catalysts in the automotive industry; as an additive for flame retardant materials in the fire protection field; as an ink-absorbing coating in high-grade inkjet printing paper in the papermaking industry; as a gas purification adsorbent, drinking water defluoridation agent, and industrial wastewater color and odor eliminator in the environmental protection industry; as a coating additive in the construction industry; and as a reinforcing agent in ceramic composite materials.
二. Relationships and differences among the three
1. Relevance
Phase transition diagram of alumina and its hydrate
Alumina has many isomorphous phases. During the sintering process of alumina hydrate, metastable phases such as χ, ρ, γ, κ, δ, θ and α may appear. Pseudoboehmite is a kind of incompletely crystallized boehmite. γ-Al2O3 calcined between 400 and 700℃ is widely used as a catalyst carrier, catalyst and adsorbent; γ-Al2O3 calcined between 1100 and 1200℃ is used in high-end ceramics, petrochemicals and other fields.
Pseudoboehmite can be converted into boehmite under specific hydrothermal conditions, realizing a one-way conversion between the two. In addition, pseudoboehmite and boehmite are both precursors for the preparation of alumina.
2. Differences
Pseudoboehmite has the same molecular formula (AlOOH) as boehmite, and they are different crystal forms of monohydrated aluminum oxide. The reason why it is called pseudoboehmite is that pseudoboehmite is a kind of boehmite with incomplete crystallization. They have very similar XRD patterns, and the positions of the main diffraction peaks of the two are almost the same. The only difference is the intensity of the diffraction peaks. Therefore, compared with pseudoboehmite, boehmite is just larger in grains, higher in crystallinity and more complete in crystallization.
XRD spectra of pseudo-boehmite and boehmite
Research suggests that there is no clear dividing line between the two. It is usually more appropriate to use the average grain size as the main criterion to divide boehmite and pseudo-boehmite. Products with an average grain size less than 10nm are considered pseudo-boehmite; products with an average grain size greater than 50nm are considered boehmite; when the average grain size is 10-50nm, it is considered an intermediate product between boehmite and pseudo-boehmite, and the intermediate product can also be classified as pseudo-boehmite.
In summary, the differences between the two are as follows:
(1) Pseudo-boehmite is an aggregation of amorphous substances with only weak diffraction peaks; boehmite has a better crystal structure and a sharp diffraction peak.
(2) Pseudo-boehmite has a higher specific surface area and a larger pore volume than boehmite. The specific surface area of pseudo-boehmite gradually decreases with increasing temperature, while the specific surface area of boehmite first increases and then decreases with increasing temperature.