# Development of Effective Infrared Reflective Coatings
## 1 Introduction
In order to compare the usefulness of infrared reflective coatings with other possibilities that can also reduce heat absorption and improve energy efficiency, we have to mention other resolutions like:  
* Low-emissivity coatings: these can reflect and absorb infrared and block UV radiation. They effectively control the heat exchange between the interior and exterior of buildings, but an infrared reflective coating reduces heat absorption [42,43]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**General Description** & **Materials in the Coating** & **References** \\ \hline Metals &
\begin{tabular}{c} Aluminum, aluminum foil, \\ metallized thin films \\ \end{tabular} & [13, 14, 15] \\ \hline Metal oxides & TiO\({}_{2}\), ZnO, CuO & [16,17] \\ \hline \multirow{6}{*}{Inorganic pigments} & Fe\({}_{2}\)O\({}_{3}\), Cr\({}_{2}\)O\({}_{3}\) ZnS, \\  & Sb\({}_{2}\)O\({}_{3}\), ZrO\({}_{2}\), SrCuSi\({}_{4}\)O\({}_{10}\), \\ \cline{1-1}  & Nd\({}_{2}\)Si2O\({}_{7}\), \\ \cline{1-1}  & BiVO\({}_{4}\) \\ \cline{1-1}  & Cr\({}_{2}\)O\({}_{3}\), BiFeO\({}_{3}\), Ce4 Si\({}_{3}\)S\({}_{12}\) \\ \hline Composites: oxide-metal oxide & TiO\({}_{2}\)/Ag/TiO\({}_{2}\), ZnS/Ag/ZnS & [19] \\ \hline Single layers and stacked polar & SiC, GaN & [20] \\ \hline Single and multilayers & Ag doped hafnium nitride & [21,22] \\ \hline Small spheres & glass, glitter, crystals & [16,22, 23, 24, 25] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Infrared reflective pigments used in the coatings. * Thermochromic coatings: These change their color and/or their opacity due to altered temperature; in this way, they control the amount of heat that enters buildings. But, as mentioned in the previous case, infrared reflective coatings reduce heat absorption more significantly [44; 45]. * Phase-change materials: some materials, when they change their phase from liquid to solid, absorb or release heat, regulate temperature, and reduce energy consumption in a building. Though the final effect of these materials and infrared reflective coatings are similar, the mechanisms for how they act and where they can be used are different. The application of an infrared reflective coating is more extensive [46; 47]. The selection of the best infrared reflective coating needs systematic screening. The pigments should be compatible with any other ingredients in a coating; they could be used in combination with different pigments. The particle size of pigments has an important impact on the refractive activity (they must be in the range between 0.35 and 0.55 \(\upmu\)m). The cost, compatibility, performance, and aesthetics, as well as environmental impact and energy efficiency, determine the applicability of infrared reflective pigments. These pigments could have the following arrangements:  
* One-layer system: the coating is applied directly onto a solid substrate, which contributes to an increase in the refractivity of the coating [18; 48]. * Two-layer system: this is the more often used technique; a top coat with infrared transparency contains the proper pigment and is deposited onto a primer. The sun's radiation influences first the upper layer [49]. * Multilayer structure: multiple layers with different refractive properties are deposited onto each other. The interference effect between the layers increases their reflective properties. It is important to take into consideration not only the chemical properties but also the thickness of each layer [50]. Generally in the case of paints, the lifespan of an infrared reflective coating depends on different factors like UV radiation, surface preparation, moisture, chemical resistance, etc. The proper selection of pigments and ingredients can enhance the lifespan of the coated surfaces and decrease energy consumption. The aim of this work was to elaborate on infrared reflective as well as transparent coatings. The importance of these types of coatings has already been detailed.