# Progress in Transparent Nano-Ceramics and Their Potential Applications
## 4 Transparent Armor Nano-Ceramics
### Alon  
AlON transparent nano-ceramics have excellent light transmittance in the near-ultraviolet to mid-infrared band (0.2-6 \(\upmu\)m), and the theoretical transmittance is as high as 85.2% [84]. In addition, they also have excellent properties, such as high hardness, high strength, high-temperature resistance, friction resistance, and acid and alkali corrosion resistance. In 1959, [PERSON] and [PERSON] [85] first reported cubic spinel-type aluminum oxynitride (AlON), which was produced from a compound between alumina and aluminum nitride in a reducing atmosphere above 1650 \({}^{\circ}\)C. They also studied the physical constants of AlON, such as crystal structure, density, refractive index, dielectric constant, and magnetic susceptibility. First, nano powder is required to fabricate AlON transparent nano-ceramics. After obtaining the AlON nano powder, it should be fabricated by sintering at a temperature above 1850 \({}^{\circ}\)C for at least 20 h in nitrogen. In 2011, [PERSON] et al. [86] adopted a two-step approach with Al\({}_{2}\)O\({}_{3}\) and aluminum nitride (AlN) nano powders as the starting materials to prepare AlON transparent nano-ceramics. In their studies, the average particle size of nano powders of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\), \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), and AlN were 80, 20, and 20 nm, respectively. Experiments showed that the AlON Nano-ceramic samples were transparent after sintering at 1880 \({}^{\circ}\)C for more than 5 h. With the extension of the holding time, the grain size of the sample increased slightly, while the pore size and porosity decreased obviously, so the light transmittance increased. Additionally, the sample had a transmittance of 55% (near 5 \(\upmu\)m band) at a hold time of 20 h. While the infrared transmittance of ceramics is promising, the visible transmittance is not high enough. In 2012, [PERSON] et al. [87] fabricated a highly AlON transparent nano-ceramic that was sintered from nano powder without pressure. During carbothermal nitridation, a layer of amorphous carbon on the surface of Al\({}_{2}\)O\({}_{3}\) particles effectively prevented agglomeration and grain growth, and the bimodal particle size distributions of the obtained AlON powders were concentrated at 200 and 700 nm, and their maximum particle sizes were both below 900 nm. The experimental results confirmed that the AlON transparent nano-ceramics with an average online transmittance above 80% in the visible light to infrared range were obtained by a pressureless sintering method. In 2018, [PERSON] et al. [88] carried out a detailed investigation of planetary ball-milling for coarsened AlON nano powder. Their results revealed that the weight ratio of balls to powder, rotational speed, and planetary milling time had a significant effect on the microscopic morphology, particle size distribution, and average particle size of the powders. Using fine and uniform AlON nano powder with an average particle size of less than 300 nm and excellent sintering properties, subsequently, a sample of AlON was successfully fabricated from the finely treated powder synthesized by the carbon thermal nitriding method at 1880 \({}^{\circ}\)C for 6 h. The optical test demonstrated that the high performance with an online transmittance of 84% at 2000 nm of the sample could be achieved. In 2022, [PERSON] et al. [89] thoroughly milled the AlON powder to fully mix the nanopowder with the sintering agent. Figure 12a shows an SEM image of the particle morphology after grinding; the particle size distribution is shown in Figure 12b,c, which shows that the high-temperature decarbonization and ball-milling process do not affect the purity of the AlON powder. In the preparation of AlON transparent nano-ceramics, sintering aids are generally used to control the growth of Al\({}_{2}\)O\({}_{3}\) grains. MgO and Y\({}_{2}\)O\({}_{3}\) were used as co-sintering aids by [PERSON] et al. [90] to fabricate AlON transparent nano-ceramics by reactive sintering.