The Research And Application Of Microporous Composites For Phase Change Material

Microporous materials possessed a lot of advantages, such as, vast surface area and abundant pore volume, which was filled with phase change materials (PCMs). It was called for microporous composites as form-stable PCMs for thermal energy storage that can solve some problem in traditional PCMs, for example, low energy and leaking easily. In order to improve the thermal storage performance and stability, the form-stable PCMs were prepared using n-carboxylic acids as thermal storage materials and microporous materials as porous supporting medium by the method of impregnating preparation.Part I, conjugated microporous polymers (CMPs) were used as porous supporting materials to prepare form-stable PCMs composites, which were syntheticed by monomers with the superhydrophobicity/superoleophilicity such as1,3,5-ethynylbenzene. The morphological structure and thermal performance of the form-stable PCMs composites were studied by using the fatty acid with difference of the molecular size. The CMPs should be ideal porous materials as the carrier for large Brunauer-Emmett-Teller (BET), for instance, CMP-2was calculated689m2g-1. Due to the unique superoleophilicity of CMPs with a water contact angle of151°, melted PCMs could be absorbed spontaneously into CMPs and can stay stable in the CMP samples without leakage even over their melting points. Results obtained from X-ray powder diffraction (XRD) show that the peak position of PA in the composite does not change, however, the intensity became lower compared with that of pure PA. The latent heat of CMP/PCM composites was measured ranging from103.3kJ kg-1to171.0kJ kg-1by the differential scanning calorimetry (DSC). In our study, the CMP-2/palmitic acid (PA) composites as PCMs exhibited expected properties, the phase transition temperature was determined to be65.3℃and53.2℃and the transition enthalpy was calculated to be171.7kJ kg-1and177.7kJ kg-1respectively during the melting and crystallization. It possessed good thermal cycling performances that phase transition enthalpy of CMP-2/PA still was measured to be179.6kJ kg-1and182.9kJ kg-1respectively even after300times cycles. The conclusion showed that CMP-2/PA composites form-stable as PCMs has excellent properties, which make them promising candidates as heat storage materials for application.Part Ⅱ, graphene has recently been reported to possess unique electrical, optical and thermal transport properties. Porous graphene (PG) was synthesised by the hard templates. The form-stable PG/PCM composites were prepared using PA as thermal storage materials and PG as supporting medium. The porous graphene of0.5nm to5nm in diameter were synthesized using calcium carbonate (CaCO3) micro-spheres as hard templates, which exhibited comparably surface superhydrophobicity with a water contact angle of152.3°by chemical vapor deposition (CVD) method using polydimethylsiloxane (PDMS). The porous graphene modified by PDMS was used as porous supporting material to prepare the form-stable phase change material by impregnation using palmitic acids as thermal storage material, the content of adsorbed palmitic acid was80.2%, and the pores of the porous graphene could be completely filled. The results obtained from XRD and DSC showed that the incorporation of PA did not change the crystal structure of the phase change material, and latent heat of melting and crystallizing can be up to167.8kJ kg-1and170.1kJ kg-1respectively. The phase change material has better performance for the areas of energy saving applications and solar energy storage.Part III, nickel foam was choosed as porous skeleton material in order to expand the selection scope of phase change of medium. Superoleophilic graphene-nickel foam (PDMS-G-NF), which was prepared from facile coating of graphene nanosheets onto nickel foam followed by surface modification with polydimethylsiloxane, was used as porous supporting materials to prepare form-stable PCMs composites. Graphene covered evenly on the surface of nickel foam, which be concluded by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Owing to the strong hydrophobic and superhydrophilic wettabilities of PDMS-G-NF, no leakage of PCM was observed from PDMS-G-NF even after heating PDMS-G-NF/PA composites beyond their melting point. PCMs could be absorbed spontaneously into PDMS-G-NF and can stay stable without leakage during the melting process. The melting and freezing latent heat was calculated to be126.34kJ kg-1and123.41kJ kg-1of PDMS-G-NF, which higher148%and152%than NF/PA. In addition, the resulting PCMs composites are much easier to prepare and handle for practical use.Taking advantages of simple and superoleophilic process, the prepared microporous composites as form-stable PCMs not only show good thermal stability and high latent heat, but also simple process steps of preparation and exhibit excellent recyclability, which make them a promising candidate to improve energy utilization.