Scientists at Donghua University in Shanghai, home to the State Key Laboratory of Advanced Fiber Materials, have developed transparent polymethylsilsesquioxane (PMSQ) aerogels that combine ultralow thermal conductivity with exceptional optical clarity. The work, published in Nature Communications, describes how carefully controlled sol–gel chemistry enables printable inks that yield aerogels with 97% transmission of visible to near-infrared light and a thermal conductivity of 16.2 mW m⁻¹ K⁻¹, lower than still air.
The approach uses methyltrimethoxysilane (MTMS) as the precursor. MTMS generates PMSQ, an inherently hydrophobic material that resists water absorption, an important property for long-term insulation in humid environments. Regulation of the sol–gel process is achieved by dual modulators: acetic acid and urea. Acetic acid, a small volatile molecule, lowers the pH to around 4 and accelerates hydrolysis. Urea thermally decomposes into ammonia and carbon dioxide, shifting the pH upward and activating condensation. By reintroducing acetic acid at critical moments, condensation can be slowed, stabilizing viscosity. This “activation–retardation” cycle produces inks with tunable rheology.
Cetyl-trimethyl-ammonium chloride (CTAC) further improves ink stability. The surfactant suppresses phase separation by balancing interactions between methyl groups and alkyl chains, and it increases viscosity to maintain filament shape after extrusion. However, the researchers note that excessive CTAC reduces crosslinking density, causing printed structures to collapse. Timing of acetic acid addition allows viscosity to be adjusted: early addition yields lower viscosity inks for bulk printing, while later addition creates higher viscosity inks capable of supporting long-span structures. Rheological tests showed that the inks display shear-thinning behavior, flowing through micronozzles under stress but rapidly recovering viscosity to retain shape. Shelf life tests confirmed stability for up to a week at 25 °C and 40% relative humidity.
After direct ink writing, the printed gels were aged at 60 °C, solidified by ammonia released from urea decomposition, and supercritically dried with CO₂. Characterization confirmed the resulting objects were pure PMSQ. Infrared spectroscopy and solid-state ²⁹Si NMR verified chemical composition, while scanning electron microscopy revealed filaments with smooth cylindrical surfaces that fused at intersections. Transmission electron microscopy showed uniform PMSQ nanoparticles with diameters around 5 nanometers, arranged into networks with average pore sizes of 23 nanometers. Nitrogen sorption analysis measured a specific surface area of 778 m²/g. Small-angle X-ray scattering produced a mass fractal slope of –1.9, characteristic of PMSQ aerogels.
Optical performance was confirmed by direct-hemispherical transmittance measurements. A 1.7 mm-thick aerogel slab transmitted 98–95% of visible light, significantly higher than commercial borosilicate glass (~90%). Near-infrared transparency was maintained due to the hydrophobic methyl groups that block moisture absorption. Mid-infrared opacity between 8–14 μm reduced radiative heat transfer. Tests with a 650 nm laser beam showed negligible deflection, consistent with the material’s refractive index approaching that of air. Rayleigh–Gans scattering theory predicts that pores larger than the wavelength of visible light scatter strongly, but the PMSQ aerogels’ 23 nm pore size avoids this effect.
Thermal properties were equally striking. Steady-state heat flow meter measurements gave a conductivity of 16.2 mW m⁻¹ K⁻¹, lower than the 25 mW m⁻¹ K⁻¹ of air. The small pore sizes suppress convection, while the low-density network minimizes conduction through solids. Thermogravimetric analysis showed stability below 350 °C. Water contact angle measurements of 151° confirmed superhydrophobicity, attributed to methyl groups throughout the PMSQ structure.
Demonstrations included architectural, electronic, and agricultural uses. Aerogels were 3D printed into pyramid skylights that maintained transparency while offering insulation. Capacitors capped with PMSQ aerogels stabilized at 48 °C under heat load, compared with 80 °C without protection. Lampshades produced from PMSQ passed visible and near-infrared light to sustain photosynthesis while blocking excess heat. Infrared images showed bulbs wrapped in PMSQ shades transmitted light with minimal heat, outperforming cotton or poly(methyl methacrylate) (PMMA) controls. The aerogels also transmitted up to 94% of ultraviolet light at 1.7 mm thickness, supporting plant growth processes such as photomorphogenesis and secondary metabolite production.
Earlier attempts at 3D printed aerogels used polymer-modified sols or aerogel powder slurries. These methods left residual organics or required ammonia vapor, which caused uncontrolled gelation and opacity. By contrast, Donghua University’s process yields pure PMSQ aerogels with reproducible rheology and transparent microstructures. Direct ink writing enabled objects with 160 μm resolution and periodic lattices that retained shape after drying. Importantly, the researchers note that the activation–retardation approach could also be used for mold processing, avoiding structural damage during demolding that often occurs with traditional methods.
“The value of our work lies in the activation–retardation process in sol–gel reactions, and the viscosity of the obtained gel ink is adjustable and can remain stable under ambient conditions,” the authors state. They emphasize that beyond complex architectures, the approach allows simple slabs that function as thermal barriers for windows or solar collectors. Laboratory tests with aerogel panes in simulated glazing systems demonstrated enhanced insulation, while solar thermal collector prototypes illustrated efficient heat capture with low optical loss.
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Featured image shows 3D printing of transparent aerogels by direct ink writing. Image via University of Donghua.
