Designing Nanoporous Polymer Films for High-Performance Passive Daytime Radiative Cooling.

Energy-free passive daytime radiative cooling (PDRC) technology makes it an attractive solution to both the building energy crisis and global warming. Spectrally selective porous polymers have great potential for practical PDRC applications owing to their cooling performance and scalability. A fundamental understanding of the relationship between the cooling performance and pore properties is crucial for guiding future structural designs of high-performance PDRC materials. However, one of the key challenges is achieving uniform nanopores and tailorable pore morphologies in the PDRC coating films. Here we demonstrate a strategy to use advanced metal-organic framework (MOF) nanocrystals as a sacrificial template creating a nanoporous poly(vinylidene fluoride) (PVDF) coating film with uniform-sized nanopores for highly daytime passive radiative cooling. The experimental evidence indicates that nanopores around 400 nm in size, comparable to the wavelength within the ultraviolet and visible spectra, along with an appropriate porosity of 37%, contribute to excellent solar reflectance (94.9 ± 0.8%) and high long-wave infrared emission (92.8 ± 1.4%) in the resulting porous PVDF films. This leads to subambient cooling of ≈9.5 °C and a promising net cooling power of 137 W/m2 at midday under solar intensities of ∼1275 and ∼1320 W/m2. The performance equals or exceeds that of state-of-the-art polymeric PDRC designs, and this general strategy of tailing nanostructures is expected to open a new avenue toward high-performance radiative cooling materials for PDRC applications.

Damage-tolerant material design motif derived from asymmetrical rotation.

Motifs extracted from nature can lead to significant advances in materials design and have been used to tackle the apparent exclusivity between strength and damage tolerance of brittle materials. Here we present a segmental design motif found in arthropod exoskeleton, in which asymmetrical rotational degree of freedom is used in damage control in contrast to the conventional interfacial shear failure mechanism of existing design motifs. We realise this design motif in a compression-resisting lightweight brittle material, demonstrating a unique progressive failure behaviour that preserves material integrity with 60-80% of load-bearing capacity at >50% of compressive strain. This rotational degree of freedom further enables a periodic energy absorbance pattern during failure yielding 200% higher strength than the corresponding cellular structure and up to 97.9% reduction of post-damage residual stress compared with ductile materials. Fifty material combinations covering 27 types of materials analysed display potential progressive failure behaviour by this design motif, thereby establishing a broad spectrum of potential applications of the design motif for advanced materials design, energy storage/conversion and architectural structures.

Capillary bridges between unsaturated nano-mineral particles: a molecular dynamics study.

Capillary bridges play an important role in the process of cohesion, which is crucial for wet granular media, and engineering of pharmaceuticals and food processing. However, the understanding of capillary bridges at the nanoscale remains unclear because the mechanical performance of nanoscale capillary bridges cannot be fully captured and explained by classical capillary theory. We applied a novel molecular dynamic simulation to investigate the dynamic formation process of nanoscale capillary bridges between quartz asperities. In comparison with classical capillary theory, our results suggested that the application of the toroidal approximation and gorge method will break down at the scale of 1 nm. Below this threshold, a pronounced oscillation in the adhesive force was observed due to inconsistent distribution of water molecules in the capillary bridges. Moreover, we found a non-linear correlation between the adhesive force and the saturation degree. Different from the cohesive stress of sandy soil as a function of saturation degree, we identified an optimal saturation range of 0.5-0.7 instead of 0.2-0.9 for the sandy soil. Our findings enhance the understanding of capillary bridges and provide new insights into the capillary force between particles in the fields of geotechnical engineering, food-process engineering, the pharmaceutical industry and nanotechnology.

Properties of wastepaper sludge in geopolymer mortars for masonry applications.

This paper presents the results of an investigation into the use of wastepaper sludge in geopolymer mortar systems for manufacturing construction products. The investigation was driven by the increasing demand for reuse options in paper-recycling industry. Both fresh and hardened geopolymer mortar properties are evaluated for samples incorporating dry wastepaper sludge, and the results indicate potential end-use benefits in building product manufacture. Addition of wastepaper sludge to geopolymer mortar reduces flow properties, primarily due to dry sludge absorbing water from the binder mix. The average 91-day compressive strength of mortar samples incorporating 2.5 wt% and 10 wt% wastepaper sludge respectively retained 92% and 52% of the reference mortar strength. However, contrary to the normal trend of increasing drying shrinkage with increasing paper sludge addition to Portland cement matrices, the corresponding geopolymer drying shrinkage decreased by 34% and 64%. Equally important, the water absorption of hardened geopolymer mortar decreased with increasing paper sludge content at ambient temperatures, providing good prospects of overall potential for wastepaper sludge incorporation in the production of building and masonry elements. The results indicate that, despite its high moisture absorbance due to the organic matter and residual cellulose fibre content, wastepaper sludge appears compatible with geopolymer chemistry, and hence serves as a potential supplementary additive to geopolymer cementitious masonry products.

Reuse of de-inking sludge from wastepaper recycling in cement mortar products.

This paper presents results of an investigation into the use of de-inking sludge from a paper recycling mill as feedstock material in the manufacture of cement mortar products, including masonry blocks and mortar renders. Both physical and mechanical properties of mortar specimens containing various amounts of de-inking sludge were investigated. It was observed that the addition of de-inking sludge to cement mortar at a fixed water-to-cement ratio significantly reduced flow properties and increased setting time. Water absorption and volume of permeable voids of cement mortar increased with increased dosage of de-inking sludge, with a corresponding reduction of bulk density. The 91-day compressive strength of mortar samples with 2.5 wt% and 20 wt% de-inking sludge loadings retained 83% and 62% respectively of the reference mortar strength. The corresponding drying shrinkage increased by up to 160% compared to reference samples. However, a de-inking sludge loading of up to 2.5 wt% did not significantly alter measured physical and mechanical properties. The results demonstrate that despite the high moisture absorbance of de-inking sludge due to its organic matter and residual cellulose fibre content, it serves as a potential supplementary additive and its cellulosic content proving to be an active set retardant to cementitious masonry products.

Surface modification for stability of nano-sized silica colloids.

The surfaces of commercial 30-nm colloidal silica particles were modified by reacting with functional silanes. The high specific surface area and reactivity of the particles due to the small size make the process susceptible to irreversible aggregation not found previously with larger particles. This study compares surface charge results from different reaction conditions and characterization methods. Measurements of the zeta potential as a function of pH and gelation kinetics shed light on the mechanism of instability in nano-sized silica suspensions. Experimental results showed that very stable particles can be suspended in a nonaqueous solvent after refluxing of the silica particles, while maintaining the original particles physical properties of size and electrochemical behavior. Extremely stable particles are obtained by aminosilane surface modification. Factors affecting susceptibility of small particles to irreversible aggregation caused by a nonaqueous solvent or a high concentration of a trialkoxysilane, including the large amount of reactive silanol groups on the surface gel layer of the particles, are discussed.