Engineering the optical properties of SrZrO3 crystals via Zn doping for superior passive daytime radiative cooling

Materials scientists have long sought solutions to mitigate the escalating energy crisis and environmental impact driven by rising global energy consumption, particularly the significant portion attributed to traditional refrigeration systems. The inherent drawbacks of these systems—high electricity demand and environmentally damaging refrigerant leakage—necessitate energy-free alternatives. Passive daytime radiative cooling (PDRC) materials offer a promising pathway by simultaneously reflecting solar radiation and emitting heat through the atmospheric transparency window. Among potential candidates, strontium zirconate (SrZrO₃) perovskite ceramics stand out due to their intrinsic properties: a wide bandgap enabling high UV-VIS reflectivity, and strong phonon-polariton resonances within the 8-14 μm atmospheric window yielding high thermal emissivity, alongside exceptional UV resistance and thermal stability. However, the full potential of SrZrO₃ for PDRC applications is constrained by limitations in its solar reflectivity and atmospheric window emissivity, primarily stemming from its wide bandgap affecting refractive index and localized phonon vibration modes. Strategic ion doping, particularly with Zinc (Zn²⁺), emerges as a viable approach to overcome these limitations by modulating the bandgap/refractive index to enhance solar reflection and inducing lattice distortion to broaden phonon vibration patterns for improved thermal emission. Despite the recognized potential of Zn-doping strategies in modifying various material properties and the theoretical prediction of its effects on SrZrO₃, the comprehensive experimental and theoretical investigation of how Zn²⁺ doping specifically and synergistically refines the phase structure, electronic structure, spectral radiative characteristics, and ultimately the practical PDRC performance of SrZrO₃ ceramics has yet to be systematically conducted. Filling this research gap is imperative for the rational design of high-performance, durable PDRC materials.

Recently, a research team led by Wen Ma synthesized a series of Zn-doped SrZrO₃ (SZZO) ceramics via sol-gel and solid-phase synthesis. This work not only experimentally demonstrates that optimal Zn doping (e.g., SrZr_0.75Zn_0.25O_2.75) significantly enhances solar reflectivity (reaching 0.892) and atmospheric window emissivity (reaching 0.954), leading to substantial sub-ambient radiative cooling temperatures (15.3 °C) and net cooling power (64.7 W·m^-2), but also elucidates the underlying mechanisms—linking improved solar reflection to modified grain morphology and electronic structure, and enhanced thermal emission to lattice distortion and reduced symmetry—through combined experimental characterization and theoretical calculations.

The team published their work in Journal of Advanced Ceramics on July 25, 2025.

“In this report, we synthesized Zn-doped SrZrO₃ powders by sol-gel method combined with solid-phase synthesis. Then, Zn-doped SrZrO₃ crystal’s phase structure, micromorphology, oxygen vacancy changes, and spectral radiative properties—including the reflection properties in the 0.3-2.5 μm band and the radiative properties in the 8-13 μm band—were studied. The effect of electronic structure changes on the optical properties of doped crystals was also investigated,” said Wen Ma, professor at Inner Mongolia University of Technology, a senior expert whose research interests focus on advanced ceramic materials and coatings.

“Zn-doped SrZrO₃ has been identified as a promising material for passive daytime radiative cooling. However, comprehensive studies linking its microstructural evolution to spectral performance optimization were lacking. For functional ceramics, it is critical to understand these structure-property relationships,” said Wen Ma.

All Zn²⁺ ions completely entered the SrZrO₃ lattice, substituting Zr⁴⁺ while maintaining uniform elemental distribution and the original orthorhombic perovskite structure. With increasing Zn doping, ZnO’s sintering effect transformed grain microstructures from independent grains to fine rods, coarse rods, small sheets, and large sheets. This evolution enhanced the specular reflectance for 0.3-2.5 μm photons. Although Zn doping reduced the band gap (still >5.5 eV), the slightly narrower gap increased the refractive index in the solar band, further boosting 0.3-2.5 μm reflectance. Concurrently, lattice distortion and reduced symmetry from oxygen vacancies enhanced 8-13 μm band emissivity. “The synergistic improvement in both solar reflectance and atmospheric window emissivity—reaching 0.892 and 0.954 for SrZr_0.75Zn_0.25O_2.75—confirms its potential as a radiative cooling material,” said Wen Ma.

Passive daytime radiative cooling coatings using SrZr_0.75Zn_0.25O_2.75 and SrZrO₃ powders were fabricated by scraping. Under 503 W·m^-2 solar irradiance, the SrZr_0.75Zn_0.25O_2.75 coating achieved 9.1°C cooling (versus 7.8 °C for SrZrO₃). At 600 W·m^-2, a 100 μm-thick coating cooled 9.8°C (versus 9.1°C at 70 μm), with no significant gain beyond 130 μm. Under 654 W·m^-2, the 100 μm coating reached 15.3 °C cooling and 64.7 W·m^-2 net cooling power. “The exceptional cooling performance and stability—maintained after 21 days of weathering—demonstrate SrZr_0.75Zn_0.25O_2.75’s viability for practical radiative cooling applications,” said Wen Ma.

However, more systematic studies are needed to advance Zn-doped SrZrO₃ coatings for real-world implementation. In this regard, Ma also proposed five key research directions: long-term environmental stability (humidity/UV resistance), substrate adhesion optimization, scalable coating deposition techniques, performance under cyclic thermal loading, and field testing in diverse climatic conditions.

Other contributors include Yangyang Li, Yichuan Yin, Yu Bai, Yuanming Gao from the School of Materials Science and Engineering at Inner Mongolia University of Technology in Hohhot, China; Hongying Dong from the School of Chemical Engineering at Inner Mongolia University of Technology in Hohhot, China; Ting Yang from the Inner Mongolia Academy of Science and Technology in Hohhot, China.

This work was supported by the Science and Technology Projects of Inner Mongolia Autonomous Region (2024SKYPT0011, 2022ZD02, 2022MS05003), Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (NJYT22080, NMGIRT2319), Basic Scientific Research Expenses Program of Universities Directly under Inner Mongolia Autonomous Region (JY20220041, JY20220062), and The Industrial Technology Innovation Program of IMAST (2024RCYJ02007). The authors are grateful to Dr. Xueping Zhao and Dr. Xiaohu Hou from the analysis and test center of Inner Mongolia University of Technology, Dr. Xiaoyan Liu from School of Materials Science and Engineering of Inner Mongolia University of Technology, and Prof. Guoqiang Xu and Prof. Ting Wang from School of Architecture of Inner Mongolia University of Technology for their testing help.

About Author

Corresponding author: Prof. Wen Ma, Ph.D. supervisor, received his Doctorate in Engineering (Materials Science) from Beihang University in 2006. From 2006 to 2007, he conducted postdoctoral research at Forschungszentrum Jülich in Germany. He is currently the Director of the v.

Prof. Ma has been honored with several prestigious awards and titles, including the "New Century Excellent Talents" by the Ministry of Education, the "Grassland Talent" award, "Outstanding Contribution Expert of Inner Mongolia Autonomous Region", "Outstanding Young Scientific and Technological Leader of Universities in Inner Mongolia", and "Excellent Science and Technology Worker of Inner Mongolia".

He serves as the Deputy Director of the National Technical Committee for Standardization of Metallic and Non-metallic Coatings, a council member of the 7th Council of the Chinese Society for Surface Engineering, and the Deputy Director of the 7th Casting Alloys Professional Committee of the Chinese Rare Earth Society.

His research mainly focuses on the development of rare-earth ceramic thermal barrier coating materials, as well as the innovation and application of advanced coating preparation technologies. In recent years, as the principal investigator, he has led four projects funded by the National Natural Science Foundation of China (NSFC), along with several key scientific and technological projects at the provincial and ministerial levels, including major projects in Inner Mongolia.

He has published over 100 academic papers, more than 70 of which are indexed by SCI. He has applied for 39 national invention patents, with 23 granted. He received the First Prize of Natural Science of Inner Mongolia in 2016.

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC’s 2024 IF is 16.6, ranking in Top 1 (1/33, Q1) among all journals in “Materials Science, Ceramics” category, and its 2024 CiteScore is 25.9 (5/130) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508