Electrospinning Machine | Superdurable, Flexible Ceramic Nanofibers for Sustainable Passive Radiative Cooling

Research Background

Global warming triggers many environmental challenges such as sea-level rise, heatwaves, and wildfires. The increasing frequency and intensity of heatwaves exacerbate wildfire risk, while electricity demand for systems like air conditioning rises sharply, expected to triple by 2050. Passive Daytime Radiative Cooling (PDRC) systems, as an innovative passive cooling technology, reduce solar heat gain by enhancing sunlight reflectivity and promote heat dissipation to outer space through mid-infrared (MIR) emissivity (especially within the atmospheric transparent window), promising to alleviate the energy demand growth triggered by the climate crisis. 

Existing PDRC materials have shortcomings: early photon coolers based on inorganic materials have complex and expensive manufacturing processes, making large-scale production difficult; polymer-based thermal emitters, though simple, economical, and scalable, are susceptible to issues like UV degradation, yellowing, and flammability in long-term practical environmental use; some ceramic PDRC emitters have limitations such as brittleness, insufficient flexibility, and inadequate temperature resistance. Therefore, developing PDRC materials with excellent cooling performance, long-term durability, and toughness in extreme environments is of great significance.

Recently, Professor Der-Hsien Lien's team at National Tsing Hua University, Taiwan, introduced a super-durable, flexible ZrO₂-Al₂O₃ nanofiber (sh-ZANF) membrane for sustainable passive radiative cooling. This membrane, fabricated via electrospinning combined with fluorine-free surface modification, possesses an extremely high solar reflectivity of 97.7% and a high atmospheric transparent window emissivity of 95.6%. Under 817 W/m² solar irradiance, the optimal sh-ZANF membrane achieved sub-ambient cooling of 6.6°C, with a maximum cooling power of 125 W/m². It can cool building models, car models, and handheld cameras by 14.7°C, 16.8°C, and 11.1°C, respectively. Furthermore, this all-ceramic nanofiber can withstand temperatures exceeding 1400°C, exhibits self-cleaning properties, passed accelerated environmental aging tests, and is estimated to save over 10 MJ of energy per square meter annually while reducing CO₂ emissions by up to 27%. It is suitable for future energy-efficient and sustainable cooling strategies. The related research findings were published in the journal "ACS Nano" under the title "Superdurable, Flexible Ceramic Nanofibers for Sustainable Passive Radiative Cooling".

Material Design and Preparation

Material Selection: ZrO₂ and Al₂O₃ were selected to prepare the superhydrophobic ZrO₂-Al₂O₃ nanofiber (sh-ZANF) membrane. ZrO₂ has a high refractive index (n=2.07) in the solar band, wide band gap (5.7 eV), and extremely high melting point (2370°C), providing excellent sunlight scattering and flame retardancy; incorporating a small amount of Al₂O₃ enhances fiber flexibility and improves MIR optical performance. 

Preparation Process: The all-ceramic nanofiber membrane was prepared via a sol-gel/electrospinning process using Zr(CH₃COOH)₄, AlCl₃・6H₂O, Al(O-iPr)₃ as precursors, and poly(ethylene oxide) (PEO) as a co-spinning agent, calcined at 800°C; a fluorine-free surface modification process was employed, using dimethoxydimethylsilane (DMDMS) and tetraethyl orthosilicate (TEOS) to achieve superhydrophobicity.

Material Characteristics and Performance 

Microstructure: Exhibits a highly porous nanofiber morphology with an average fiber diameter of 404±58 nm, porosity of 93%, containing tetragonal zirconia (t-ZrO₂) and amorphous Al₂O₃ domains. Optical 

Performance: Solar reflectivity up to 97.7%, originating from strong scattering due to the high refractive index contrast (n_fiber = 2.04, n_air = 1) at the fiber/air interface. Atmospheric transparent window emissivity reaches 95.6%, attributed to phonon-polariton resonance from abundant Al-O/Zr-O bonds, and the absence of a strong Reststrahlen effect. 

Cooling Performance: Under 817 W/m² solar irradiance, the optimal sh-ZANF membrane achieved 6.6°C sub-ambient cooling with a maximum cooling power of 125 W/m²; nighttime cooling power reached 112 W/m² with a peak temperature reduction of 4.6°C. Covering a building model, car model, and handheld camera reduced their temperatures under sunlight by 14.7°C, 16.8°C, and 11.1°C respectively, also extending the camera battery life by 31%. 

Thermal Stability and Flame Retardancy: Can withstand temperatures exceeding 1400°C, can protect buildings and their occupants in fire emergencies, showing no ignition or significant degradation after direct exposure to a ~1400°C heat source for 1200 s. 

Durability: Possesses self-cleaning properties, repels various aqueous droplets; passed accelerated environmental aging tests including corrosion resistance, stain resistance, and UV resistance tests, maintaining good performance in humid, high-temperature, and strong UV environments.

Figure 1. (A) Schematic of sh-ZANF with excellent properties (passive daytime radiative cooling, ultra-light weight, weather resistance, and fire resistance), prepared via scalable electrospinning technology combined with fluorine-free hydrophobic surface modification. (B) Photo of large-scale sh-ZANF membrane. (C) SEM image of uniformly distributed sh-ZANF, average diameter 404±58 nm. (D) STEM image and EDS elemental mapping of a single typical sh-ZANF. (E-G) Photos highlighting sh-ZANF's (E) lightweight (density≈0.163 g/cm³), (F) flexibility, and (G) fire resistance. (H) Hydrophobic performance of sh-ZANF in contact with different liquids: water, milk, red wine, miso soup, chocolate milk, coffee, soy milk, milk tea, and muddy water. 

Figure 2. (A) Calculated scattering efficiency of ZANF, ZrO₂ nanofibers, Al₂O₃ nanofibers, and SiO₂ nanofibers with diameters ranging from 100 to 2000 nm in solar wavelengths (0.3-2.5 μm). (B) Cross-sectional electric field distribution from 2D-FDTD simulations for ZANF membrane (thickness = 300 μm, width = 8 μm) and its bulk counterpart (thickness = 10 μm, width = 8 μm). Incident light wavelengths (λ₁): 500, 1000, and 1500 nm. (C, D) Calculated (C) solar reflectivity and (D) mid-infrared emission spectra for ZANF, Al₂O₃, and SiO₂ nanofiber membranes with fixed thickness of 500 μm and porosity of 90%. (E, F) Calculated (E) solar reflectivity and (F) mid-infrared emission spectra for ZANF membranes with thicknesses of 10-700 μm. (G) Comparison of calculated solar reflectivity and atmospheric transparent window emissivity values for ZANF membranes of different thicknesses.

Figure 3. (A) HR-TEM image of sh-ZANF. (i) Randomly arranged crystal structure. (ii) Enlarged view of area outlined in red in (i). Areas outlined by white dashed lines are amorphous regions. (iii) Enlarged view of area outlined in blue in (ii), where (101) and (200) crystal planes of tetragonal zirconia (t-ZrO₂) can be distinguished by lattice fringes. (B) XRD patterns, (C) Raman spectra, and (D) tensile stress-strain curves of sh-ZANF and ZrO₂ nanofiber membranes. (E) Solar reflectivity and (F) mid-infrared emission spectra of sh-ZANF with different basis weights. (G) Solar absorption power, daytime cooling power, and nighttime cooling power values for sh-ZANF with different basis weights. (H) Solar absorption power at different incident angles and (I) atmospheric transparent window emissivity (εₐₜᵥ) values for optimal sh-ZANF. (J) Solar reflectivity and (K) mid-infrared emission spectra of sh-ZANF, SiO₂, poly(vinylidene fluoride) (PVDF), and poly(ethylene oxide) (PEO) nanofiber membranes. (L) Comparison of average solar reflectivity (Rₐᵥ₉), average mid-infrared emissivity (εₐᵥ₉), solar reflectivity (Rₛₒₗₐᵣ), and atmospheric transparent window emissivity (εₐₜᵥ) for different nanofiber membranes based on (J, K). (M-P) Calculated equilibrium temperature and cooling power under different non-radiative heat transfer coefficients (hₑ, 0-12 W/m²・K) for (M) sh-ZANF, (N) SiO₂, (O) PVDF, and (P) PEO nanofiber membranes during daytime at ambient temperature 303 K.

Figure 4. (A) Photo and thermal infrared image of sh-ZANF exposed to a torch flame. (B) Schematic of sh-ZANF's excellent thermal insulation properties. (C) Photos of sh-ZANF, SiO₂, and PEO nanofiber membranes during combustion tests. (D) Photos of painted steel plates with and without sh-ZANF coverage when exposed to a torch flame. Inset: Photos of steel plates after flame exposure. (E) Bending strength and modulus of steel plates in original state, bare state, and covered with sh-ZANF before and after flame exposure. (F) Photos of stainless steel house models with and without sh-ZANF coverage before and after combustion tests. (G) Solar reflectivity, atmospheric transparent window emissivity, cooling power, and solar absorption power values of sh-ZANF before and after combustion tests. (H) Comparison of operating temperature and optical properties measured in this study with related data for recently reported high-performance ceramic/polymer-based PDRC materials. (I) Comparison of basis weight and optical properties measured in this study with related data for recently reported lightweight PDRC materials.

Figure 5. (A) Photo and (B) schematic of the setup for thermal measurements. (C) Daytime and (D) nighttime temperature profiles of sub-ambient cooling performance for sh-ZANF measured in Hsinchu, Taiwan (120.98°E, 24.78°N). (E, F) Cooling power values of sh-ZANF measured during (E) daytime and (F) nighttime. (G, H) (i) Photos and thermal images of (G) building model and (H) car model uncovered (bare), covered with aluminum foil, and covered with sh-ZANF. (ii) Internal temperature changes of the building model (G) and car model (H), and corresponding temperature reductions for the two cooling materials. (I) (i) Photos and thermal images of a handheld camera uncovered and covered with sh-ZANF. (ii) Surface temperature change of the camera and corresponding temperature reduction due to covering with sh-ZANF. (iii) Battery life of cameras uncovered and covered with sh-ZANF. (J) Ambient temperature over 24 hours and (K) calculated sh-ZANF surface temperature over 12 months for the Taipei area. (L) Calculated annual energy savings using sh-ZANF for 16 major global cities, and (M) corresponding CO₂ emission reductions.

Figure 6. (A) Contact angles of ZANF and sh-ZANF samples. (B) Solar reflectivity/absorption spectra, mid-infrared emissivity spectra, and (C) contact angle of sh-ZANF before and after soaking in various pH solutions for 1 week. (D) Top: Photos of ZANF and sh-ZANF samples treated with muddy water then rinsed with water. Bottom: Photos and thermal infrared images of samples under direct sunlight. (E, F) Solar reflectivity/absorption and mid-infrared emissivity spectra of sh-ZANF before and after (E) damp heat and (F) UV pretreatment accelerated aging tests on a laboratory scale.

Conclusion

The super-durable electrospun sh-ZANF membrane has been proven to overcome the limitations of current polymer-based and ceramic-based materials in achieving sustainable passive cooling in practical applications. Theoretical and experimental spectra clearly demonstrate that the optimally designed sh-ZANF possesses passive daytime radiative cooling (PDRC) capability, with a solar reflectivity of up to 97.7%, an atmospheric transparent window emissivity of 95.6%, and a theoretical cooling power of 113.4 W/m². Furthermore, sh-ZANF can withstand a high temperature of 1407 °C under a butane flame, setting a record, which highlights its potential as a building exterior coating material. Moreover, it achieves significant cooling of 6.6 °C during the day and 4.6 °C at night, with maximum cooling powers of 125 W/m² and 112 W/m², respectively. The building model, car model, and handheld camera covered with sh-ZANF achieved significant temperature reductions of 14.7 °C, 16.8 °C, and 11.1 °C under sunlight, respectively, also extending the camera's battery life by 31%. Finally, various extreme environmental tests confirmed that sh-ZANF has strong weather resistance, indicating its suitability for long-term outdoor applications. In summary, the developed sh-ZANF can serve as a flexible, ultra-lightweight, flame-retardant, and weather-resistant radiator for various external application scenarios, mitigating global warming through efficient cooling.

Original link::https://doi.org/10.1021/acsnano.5c05958