| Energy is the basis of human existence.With the rapid development of global industry,global energy is becoming increasingly scarce.Phase change energy storage materials have good application prospects.Phase change materials(PCMs)can be applied in the storage and release of energy through the phase change latent heat during the phase change process,which can be helpful to improve energy efficiency and develop renewable energy sources.However,PCMs have many disadvantages during the phase change process,such as leakage,material exchange with the outside world,degradation during the heat absorption/release cycle,poor compatibility with the substrate,and corrosion.Micro/nano-encapsulating PCM in a polymer shell can effectively solve the above-mentioned problems to maintain its shape,improve its heat exchange capacity,and prevent PCM from leaking or decomposing during post-processing or phase change process.This encapsulated PCM must last for about 10,000 thermal cycles(or about 20 years of operational service)without any damage.However,most encapsulated commercial products have insufficient rupture strength and antiosmosis performance to tolerate volume changes or diffusional leakage during this operating period.Compared with synthetic polymers,nanocellulose has many excellent properties,such as high crystallinity,ultra-fine structure and high transparency,as well as lightweight,degradable,biocompatible and renewable characteristics of natural fibers.Due to the unique advantages of nanocellulose,there is a growing interest in nanocellulose.Therefore,nanocellulose can be utilized to reinforce polymeric materials forming a dense network resulting in sustainable,eco-friendly,and versatile polymer composites.In this dissertation,nanocellulose is used as the reinforcing phase of the microcapsule polymer wall material,which can make full use of its excellent physical and mechanical properties to prepare modified microcapsules with enhanced wall material,thereby obtaining environmentally friendly and multifunctional polymer composite materials.The main research work of this paper was as follows:(1)PCM paraffin was used as the encapsulation object,and melamine,urea,and formaldehyde were used as wall material monomers.Phase change energy storage microcapsules with proper change temperature and dense structure containing PCMs paraffin as the internal phase and melamine-urea-formaldehyde(MUF)as the external shell were synthesized by in situ polymerization.The effects of processing conditions(including core-to-shell ratio,concentration of styrene-maleic anhydride(SMA),reaction temperature,reaction time,and final reaction pH value)on the properties of synthesized microcapsules(microencapsulation process,morphology,mean diameter,and phase change properties)were systematically investigated.The chemical structures,morphologies,and phase change properties of paraffin/MUF microcapsules were characterized by Fourier transform infrared spectroscopy(FTIR),field emission scanning electron microscopy(FE-SEM),differential scanning calorimetry(DSC),and thermogravimetric analyzer(TGA).Under optimal processing conditions(core-to-shell ratio of 1.5,20 mg/m L SMA,2-h reaction at 80 ℃,and final reaction pH 5.5),the encapsulation efficiency of synthesized paraffin/MUF microcapsules reached 77.1% and the phase change latent heat values of melting and crystallization were134.3 J/g and 133.1 J/g,respectively.(2)The effects of the main emulsification variables(ultrasound time and output power of ultrasonic treatment)were explored and then paraffin/MUF nanocapsules were successfully prepared via an in-situ polymerization method.The influences of phacoemulsification parameters on the stability of the paraffin/MUF nanoemulsions and the appearances,mean droplet diameter and thermos-physical properties of paraffin/MUF nanocapsules were investigated.Scanning electron microscopy(SEM),Transmission electron microscopy(TEM),dynamic light scattering(DLS),Fourier-transform infrared spectroscopy(FTIR),thermogravimetric analyzer(TGA),and differential scanning calorimetry(DSC)were employed to characterize the morphology,size distribution,and thermal stability of paraffin/MUF nanocapsules,respectively.The characterization results showed that the more stable nanoemulsion could be formed after ultrasonic treatment for 15 min under the output power at 600 W compared with mechanical stirring emulsification.When the ultrasonic output power was 600 W,the paraffin/MUF nanocapsules achieved the highest encapsulation efficiency(35.8%)and encapsulated paraffin in the MUF shell under the output power of 600 W was more than that obtained under other ultrasonic output power values.This study verified that the ultrasound-assisted emulsification process could be used to fabricate stable paraffin/MUF nanocapsules.(3)Cellulose nanocrystals(CNCs)were used as MUF wall material reinforced additives,leakage-free and thermally stable paraffin/MUF phase change energy storage microcapsules with paraffin as the core material and MUF resin as the wall material modified with CNCs were prepared by in-situ polymerization technique.The morphological features and physicochemical characteristics of CNCs were investigated by transmission electron microscopy(TEM),zeta potential measurement,X-ray diffraction(XRD),and Fourier-transform infrared spectroscopy(FTIR).Optical microscope measurement,field emission scanning electron microscopy(FE-SEM),thermogravimetric analyzer(TGA),and differential scanning calorimetry(DSC)were employed to characterize the morphology,size distribution,and phase change properties of paraffin/MUF microcapsules.The characterization results showed that paraffin/MUF microcapsules were successfully fabricated and modified with CNCs.The CNCs had no impact on the chemical structure or crystal type of Paraffin/MUF microcapsules.When the CNC addition was 0.2 wt%,the phase change latent heat values of melting and crystallization of paraffin/MUF microcapsules were respectively104.5 J/g and 102.8 J/g and the encapsulation efficiency of which was 59.8%.(4)PCM paraffin was used as the encapsulation object,and melamine,urea,and formaldehyde were used as wall material monomers.CNCs combined with styrene-maleic anhydride(SMA)as the combined stabilizers were used to stabilize paraffin droplets for fabricating paraffin/MUF microcapsules.Effects of mixed emulsifier of CNCs and SMA on the morphology,chemical structure,and properties of paraffin/MUF microcapsules were characterized by Field emission scanning electron microscopy(FE-SEM),Fourier-transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),thermogravimetric analyzer(TGA),differential scanning calorimetry(DSC),and paraffin leakage rate test.The results showed that using CNCs alone as emulsifier did not work in the manufacturing paraffin/MUF microcapsules,but mixed emulsifier of CNCs and SMA was suitable.When CNCs and SMA each account for 50% of the mixed stabilizer,the phase change latent heat values of melting and crystallization of Micro C5S5 were 123.6 J/g and 118.4 J/g,respectively.This demonstrates that CNCs can be mixed with SMA to stabilize in situ polymerization and reduce the need for surfactants. |
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