**3.2 Aluminum-oxide: alumina (Al2O3)**

It was mentioned earlier that due to their good physical and mechanical properties, they are the most often used in the ballistic protection of soldiers and vehicles. Alumina provides excellent impact resistance, chemical resistance, abrasion resistance, and high-temperature properties and this material is cheaper than other ceramic materials for this purpose. Alumina can be found in several different phases: alpha (α), beta (β), gamma (γ), eta (η), qi (χ), delta (δ) and cap (κ) alumina [21–25]. The α-alumina particles look like white powder, have a small active surface, are more resistant to high temperatures, and have very good physical and mechanical properties. They are most often used as ceramic materials. β-alumina has a hexagonal structure with a lamellar structure. The γ-alumina particles are also nano-aluminum oxides of high purity, and also have a large active surface area. Lattices of this type of material are porous and stable at high temperatures. By modifying the structure of γ-alumina, they can be used as an adsorbent and/or catalyst [26]. η-Al2O3 particles have a similar crystal structure as γ-Al2O3 and a large specific surface area (2200 m<sup>2</sup> g−1), which is why they can be used without modification as adsorbents or as catalysts if certain modifications are made [27]. Alumina in the χ-Al2O3 phase is a metastable material of hexagonal crystal structure that has high thermal stability

and the ability to bind metal cations, which is why it can be used as a catalyst [28]. The structure of δ-Al2O3 has been studied for over fifty years, and recent research has shown that at this stage there is a complex structure created by the internal development of various polytypes from tetrahedral to octahedral structure [29]. Particles κ-Al2O3 represent one of the transition phases of aluminum oxide, which has a polyhedral crystal structure in which oxygen and aluminum usually form octahedra and tetrahedra [30]. The chemical routes of alumina powder synthesis for sintering of ballistic protective equipment can be obtained by sol–gel [31], control precipitation [32], and hydrothermal processing [33] methods.

The mass fraction of alumina in ballistic plates is generally from 96 to 99%. The following **Figure 2** shows the morphology of alumina ballistic plates. **Table 3** shows the physical and mechanical properties of alumina-based ballistic plates manufactured by CeramTec12000 (https://www.ceramtec-industrial.com/en/ ballistic-protective-ceramics).

According to the properties shown in **Table 3**, it can be concluded that with an increase in the mass fraction of Al2O3, the improvement of physical and mechanical properties is accompanied by an increase in density and mass. Impurities will mostly come from magnesium-oxide added to be avoided uncontrolled grain growth and enable the increased ballistic efficiency of corundum.
