Organopolysilazane Polymer as High Emissive Coating for Passive Daytime Radiative Cooling Applications

Abstract

Passive Daytime Radiative cooling (PDRC) technology has the potential to significantly reduce our energy dependency for cooling buildings. It is an emerging, innovative green technology which uses the deep space as a cold sink to emit excess heat from an object through earth’s atmosphere mitigating the heat island effect in urban areas. Achieving an efficient PDRC cooler requires high emission through the spectral range (8-13 μm) where the atmosphere is transparent, while reflecting the incoming solar spectra[1,2]. IR emissive polymers present a simple yet promising approach that enables controlling emission through molecular bonding[3]. However, relatively thick polymer coatings (20-200µm) are required to achieve sufficient high emissivity. To realize high cooling performance, an emitter with low absorption of the downward heat radiation from the atmosphere is desired. Aforementioned spectral selectivity on the other hand, generally requires complex nano-engineered structures to achieve restricted emission in the spectral range of 8-13µm. We have recently demonstrated for the first time a PDRC structure based on a spectrally selective organopolysilazane planar emitter which is simple to manufacture [4]. Organopolysilazane are silicon-based polymers where alternating silicon and nitrogen atoms form the backbone of the polymeric structure along with methyl and vinyl group attached to it. Polysilazane (PSZ) barrier coatings have been studied widely for their chemical resistance, stability, corrosion protection properties and easy applicability. The exceptional high transmissivity in the UV-Vis region and flexibility of polysilazane based coatings enable versatile application of these coatings[5,6]. We present here the optical properties and cooling performance of a simple dual layer spectrally selective PDRC structure with PSZ coating as a thermal emitter on a thin film silver reflector. Due to the high transparency of the polymer coating the structure allows for 97% reflection of the solar spectrum. We demonstrate the characteristic bond vibrations from the functional groups of the polymeric structure which coincides very well with the atmospheric transmittance window of 8-13µm resulting in the high emissivity of 0.9 in the 8-13µm wavelength range. The structure with a 5 µm thin coating can cool down to 6.8°C below ambient owing to a cooling power of 93.7 W/m2. Based on long term outdoor performance tests and degradation tests under different environment, the changes in optical response and reliability of the PDRC structure was evaluated. We believe that polysilazane emitter coatings due to its simplicity and tenability will open new research possibilities for PDRC applications. References- [1] A. P. Raman, M. A. Anoma, L. Zhu, E. Rephaeli, S. Fan, Nature 2014, 515, 540. [2] D. Zhao, A. Aili, Y. Zhai, S. Xu, G. Tan, X. Yin, R. Yang, Applied Physics Reviews 2019, 6, 21306. [3] A. Aili, Z. Y. Wei, Y. Z. Chen, D. L. Zhao, R. G. Yang, X. B. Yin, Materials Today Physics 2019, 10, 100127. [4] U. Banik, A. Agrawal, H. Meddeb, O. Sergeev, N. Reininghaus, M. Götz-Köhler, K. Gehrke, J. Stührenberg, M. Vehse, M. Sznajder, C. Agert, ACS applied materials & interfaces 2021, 13, 24130. [5] U. Banik, K. Sasaki, N. Reininghaus, K. Gehrke, M. Vehse, M. Sznajder, T. Sproewitz, C. Agert, Solar Energy Materials and Solar Cells 2020, 209, 110456. [6] U. Banik, N. Reininghaus, P. Seefeldt, M. Sznajder, K. Gehrke, M. Vehse, P. Spietz, T. Sproewitz, C. Agert, 2019 European Space Power Conference (ESPC) 2019.