# Progress in Transparent Nano-Ceramics and Their Potential Applications
## 4 Transparent Armor Nano-Ceramics
### MgAl\({}_{2}\)O\({}_{4}\)  
Due to its high hardness, strong chemical resistance, and high transparency in the UV-Visible and mid-IR range, MgAl\({}_{2}\)O\({}_{4}\) transparent nano-ceramics are expected to be used in optical components and defense applications [64], such as optical lenses, aircraft/vehicle windows, and missile domes [70]. Moreover, they can achieve a balance between optical performance and production cost [71]. In 1974, [PERSON] [72] fabricated translucent polycrystalline MgAl\({}_{2}\)O\({}_{4}\) ceramics with co-precipitated spinel as raw material, doped with 0.25 wt.% CaO as a sintering aid. The experimental results showed that at a certain temperature, the relative density of sintered spinel could reach 99.7% (close to the theoretical value of 100%) and that the transmittance in the visible light region was between 67 and 78%. To obtain MgAl\({}_{2}\)O\({}_{4}\) transparent nano-ceramics with high optical transparency, high-quality precursor powders (average particle size of 150 nm [73]) and optimized concentrations of sintering aids, such as CaO [72], B\({}_{2}\)O\({}_{3}\)[73], and LiF [74], are required. In 2013, [PERSON] et al. [75] fabricated MgAl\({}_{2}\)O\({}_{4}\) transparent nano-ceramics by hot pressing and investigated their characterization. They prepared two starting powder mixtures (with a purity level greater than or equal to 99.99%) made from commercial Al\({}_{2}\)O\({}_{3}\) and MgO products, taking into account the stoichiometric ratio of MgAl\({}_{2}\)O\({}_{4}\). Experimental results showed that the value of D\({}_{50}\) of MgAl\({}_{2}\)O\({}_{4}\) was close to 180 nm. The particle size of the starting powder had a slight effect on the sintering evolution and final microstructure but not uniformly on the final transmittance. Optical inspection experiments showed that up to 70% transmittance (the highest value of 78% in the 1100 nm band) was obtained in visible light. In addition, thermodynamic studies of the reactions of LiF, MgO, and Al\({}_{2}\)O\({}_{3}\) could help to understand the densification mechanism that affected the transmittance of spinel. In 2015, [PERSON] et al. [76] investigated the effect of the pressure applied during sintering  
Figure 10: Scheme of functional layers in a transparent armor concept with a four-layer design [65]. Reprinted with permission from Ref. [65]. Copyright 2017, copyright ELSEVIER. on the final optical properties of the MgAl2O4 transparent nano-ceramics in hot pressing. By establishing a thermodynamic model, the role of lithium fluoride as a sintering aid was clarified. The results of the study showed that transparency, close to the theoretical value, could only be achieved at very high pressures (200 MPa or higher) due to spinel destabilization. [PERSON] et al. [77] prepared MgAl2O4 transparent nano-ceramics from pure aluminum and magnesium sulfate using colloidal chemistry. Figure 11a,c shows that the basic particles are between 40 and 80 nm in diameter and have an isotropic shape. The phase composition of the powder is shown in Figure 11b, while the particle size distribution of the as-received powder is shown in Figure 11d. In general, it is challenging to fabricate MgAl2O4 transparent nano-ceramics using traditional pressureless sintering techniques [73; 78]. Therefore, complex sintering strategies, such as hot pressing, HIP, and SPS, are often employed to develop these ceramics [74; 75; 76; 79]. In 2010, [PERSON] et al. [80] synthesized and densified MgAl2O4 transparent nano-ceramics using the SPS method. The alumina particles in the starting powders were well separated and polygonal in shape with a particle size of about 100-500 nm, while the magnesium oxide powder was strongly agglomerated, with a size of about 10-20 \(\upmu\)m and a relatively high specific surface area. The addition of a sintering aid of 1 wt.% LiF to the mixed powder promoted the synthesis of spinel. The experimental results indicated that LiF vapor played an important role in eliminating residual carbon contamination and achieving a fully dense state.