Editing Heat shield (section)

==Principles of operation==
Heat shields protect structures from extreme temperatures and thermal gradients by two primary mechanisms.<ref>https://www.designengineering.com/content/sell/PS-Muffler-Shield.pdf</ref> [[Thermal insulation]] and [[radiative cooling]], respectively isolate the underlying structure from high external surface temperatures, while emitting heat outwards through [[thermal radiation]]. To achieve good functionality the three attributes required of a heat shield are low [[thermal conductivity]] (high [[thermal resistance]]), high [[emissivity]], and good thermal stability (refractoriness).<ref>{{cite journal | last1= Shao | first1= Gaofeng | display-authors= etal | title= Improved oxidation resistance of high emissivity coatings on fibrous ceramic for reusable space systems | journal= Corrosion Science | year= 2019 | volume= 146 | pages= 233–246 | url= https://www.sciencedirect.com/science/article/pii/S0010938X1830605X | doi= 10.1016/j.corsci.2018.11.006 | arxiv= 1902.03943 | bibcode= 2019Corro.146..233S | s2cid= 118927116 | access-date= 2019-01-11 | archive-date= 2021-10-01 | archive-url= https://web.archive.org/web/20211001050140/https://www.sciencedirect.com/science/article/abs/pii/S0010938X1830605X | url-status= live }}</ref> Porous [[ceramics]] with high emissivity coatings (HECs) are often employed to address these three characteristics, owing to the good thermal stability of ceramics, the thermal insulation of porous materials and the good [[radiative cooling]] effects offered by HECs.