CNT-Based Solar Thermal Coatings: Absorptance vs. Emittance

Abstract

A novel approach for fabricating selective absorbing coatings based on carbon nanotubes (CNTs) for mid-temperature solar–thermal application is presented. The developed formulations are dispersions of CNTs in water or solvents. Being coated on stainless steel (SS) by spraying, these formulations provide good characteristics of solar absorptance. The effect of CNT concentration and the type of the binder and its ratios to the CNT were investigated. Coatings based on water dispersions give higher adsorption, but solvent-based coatings enable achieving lower emittance. Interestingly, the binder was found to be responsible for the high emittance, yet, it is essential for obtaining good adhesion to the SS substrate. The best performance of the coatings requires adjusting the concentration of the CNTs and their ratio to the binder to obtain the highest absorptance with excellent adhesion; high absorptance is obtained at high CNT concentration, while good adhesion requires a minimum ratio between the binder/CNT; however, increasing the binder concentration increases the emissivity. The best coatings have an absorptance of ca. 90% with an emittance of ca. 0.3 and excellent adhesion to stainless steel.

Sprayable Ultrablack Coating Based on Hollow Carbon Nanospheres

“…Pure (or binder-free) low-density carbon-based materials typically provide integral absorption better than 0.99. However, these characteristics decrease significantly (about 0.02) when the nanomaterial is mixed with the binder that is required to achieve strong adhesion to the substrate in realistic applications. − A similar idea is clearly illustrated in Supporting Movie showing the reflectivity of He–Ne laser from several ultrablack coatings. Conversely, the integral absorptance of the HCS-based coatings maintains a high value of 0.985.…”

“…However, the produced coatings typically had weak adhesion to a substrate or were prepared in the form of a self-supporting membrane, thus limiting their practical applicability. To improve the adhesion of the nanotube-based coatings, Magdassi et al mixed multiwalled carbon nanotubes with a silicone, then the mixture was sprayed on an aluminum substrate to obtain the ultrablack coating. − The resulting adhesion was significantly improved, but at the cost of a substantial increase of average reflectance up to 2–5%, that is, to a level that is not acceptable for certain applications. A similar result is reported for carbon aerogels .…”

Hydrothermal Synthesis and Microwave Absorption Properties of Nickel Ferrite/Multiwalled Carbon Nanotubes Composites

“…Carbon nanotubes (CNTs), with large length to diameter ratio, low density, large specific surface area and high electrical conductivity, show good absorbing properties [11][12][13][14]. However, the high conductivity of CNTs has also led to the poor absorption caused by the mismatch of interface impedance.…”