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Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Phase change materials (PCM) has been increasingly used over the past decades to combat the large amount of energy needed for thermal comfort of human beings. Organic PCM is desired in temperature control, but it suffers from thermal leaking and unstable form during phase transition. This study aimed to develop cellulose nanofibrils (CNF) – phase change mamterials based functional materials for thermal management applications. PCM, using paraffin as an example, is encapsulated into micron-sized emulsions using CNF as interfacial stabilizer. We expected that microencapsulation using CNF can improve the flexibility and thermal stability of PCM, and such emulsion can be used in various applications such as spray coating and 3D printing. Firstly, CNF/PCM Pickering emulsion can be stabilized due to the amphiphilic nature and strong network of CNF with optimized sonication conditions, including sonication time and amplitude, ratio of CNF and PCM, and CNF concentrations. Results indicated uniform PCM emulsion particles of 4.2 ± 2.1 μm could be obtained using 0.8 wt.% CNF suspension sonicated at 100%A and 7 mins with 2:8 paraffin to CNF ratio. The CNF-stabilized paraffin emulsion showed excellent long-term stability with unchanged particle size when stored at 45 °C for 28 days. In addition, differential scanning calorimetry (DSC) results showed high thermal stability after 51 heating-cooling cycles. The CNF-stabilized paraffin Pickering emulsion demonstrated improved thermal stability and versatility for spray-coating application, which can coat a regular polyester-cotton fabric to improve its thermal shielding performance, without sacrificing its flexibility. Secondly, the CNF-stabilized paraffin Pickering emulsion can serve as a gradient to prepare CNF/paraffin printable ink for direct-ink-writing 3D printing. The rheological properties of the inks were characterized in details to demonstrate their printability. Lightweight 3D printed scaffold (35.0 mg/cm³ for pure CNF matrix) endowed the composite monolith with high PCM loading (up to 82% with respect to the composite) and high thermal storage capacity (up to 153 J/g).
This study aimed to improve the thermal management and mechanical performance of cellulose nanofibrils (CNF) aerogels. We expected that closed-pore structure of CNF aerogel can be gained by Pickering emulsion templating and solvent exchange method, and that paraffin can be encapsulated by CNF to fabricate CNF/Phase change material (PCM) aerogel with excellent stability, super thermal regulation performance, and flexibility. It was also expected that the addition of CNF to prepared emulsion can enhance the structure to further improve the thermal management and mechanical performance. First, CNF aerogels with closed internal pores were successfully fabricated by Pickering emulsion templating and solvent exchange techniques. CNF stabilized oil-in-water Pickering emulsion can be converted into closed pores by sequential solvent exchanging to acetone and tert-butanol, followed by freeze drying from tert-butanol to suppress the formation of large ice crystals. The closed pore was verified by both confocal microscopy and scanning electron microscopy images, and was confirmed to reduce thermal conductivity for aerogel. In addition to the ultra-low thermal conductivity, the CNF/emulsion composite aerogel also demonstrates high performance in properties such as low density, high mechanical properties, superb flexibility, and infrared shielding properties. In addition, robust CNF/PCM aerogels were successfully fabricated with CNF stabilized paraffin by Pickering emulsion templating technique. The CNF/PCM aerogels were ultralight with impressive mechanical properties, which were significantly enhanced by the addition of CNF to the as-formed PCM emulsion. The freeze-dried CNF/PCM aerogels were flexible, and the shape of CNF/PCM aerogel can be well maintained even loaded with a weight of over thousands of times its own weight. The exceptional thermal regulation performance was demonstrated by a series of heating and cooling tests. The CNF/PCM aerogels exhibited excellent shape stability with no leakage after 51 heating-cooling thermal cycles. The latent heat of CNF/PCM aerogel (4:6) can reach approximately 84.4% of that of pure paraffin. In brief, this work fabricated sustainable CNF aerogel with closed-pore structure for the first time, and also obtained CNF/PCM aerogel with high shape stability and energy density. The thermal management and mechanical performance of CNF aerogel were significantly improved.
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