Study On Preparation And Performance Of Palmitic Acid/ Carbonaceous Materials Phase Change Composites

Storing/releasing thermal energy during the process of phase change, phase change materials (PCMs) are ideal materials in related fields. PCMs possess of great value of improving the energy utilization efficiency and decrease the energy mismatch between time and space, contributing to solving the worldwide issues of energy shortage and environmental pollution. Correspondingly, carbonaceous materials which own the characters of abundantance, wide range of application and high thermal conductivity, whcih is deemed to be the one of most promising materials of twenty first century. Therefore, combining the carbonaceous materials and PCMs to solve the defect of the low thermal conductivity of PCMs have attract a lot of researchers’ attention.This paper choose palmitic acid (PA) which could vastly produced from agricultural and sideline products and wildly applied to solar energy storage at low temperature as the PCMs. And this paper adopting the follow methods to improve the thermal conductivity of PA:encapsulating the PA into mesopore activated carbon (AC) and adding the graphite powder with high thermal conductivity into composite to enhance the thermal conductivity; encapsulating the PA into expanded graphite (EG) with high thermal conductivity; adding carbon nanotubes (CNTs) into PA to enhance the bulk thermal conductivity. By the means of the above methods, promising to acquire the composite with proper thermal conductivity, latent heat and thermal stability.(1) Employing three-dimensional porous mesopore activated carbon (AC) as supporting material, and preparing a series of PA/AC composites with different mass ratio by physical blending and impregnating method. Concluding that the reaction between AC and PA lead to the decease of latent heat and the regulating of temperature range of phase change according to the comprehensive comparison of pore structure of AC, structure analysis of composites and DSC testing of composites. Meanwhile, The PA/AC composite with the mass ratio of 55/45 was supposed to be optimum after repeated thermal cycles of the composites. This composites keep stable and no seepage occurred after 100 thermal cycles, and it owns the latent heat of 78.2 J/g and the retention ratio reached a high level of 99.0%. Adding graphite powder into the composite at the mass ratio of 1% to prepare the PA/AC/GP composite, the latent heat is 76.9 J/g, and the thermal conductivity is 0.354 W/ m K which gain the 65.4% enhancement to PA.(2) Employing three-dimensional porous expended graphite (EG) as supporting material, and preparing a series of PA/EG composites with different mass ratio by physical blending and impregnating method. The PA/EG composite with the mass ratio of 6:1 was supposed to be optimum after repeated thermal cycles of the composites. This composites keep stable and no seepage occurred after 100 thermal cycles, and it owns the latent heat of 169.7 J/g and the retention ratio reached a high level of 98.9%. It is worth mentioning that the thermal conductivity of PA was enhanced to 2.465 W/m k from 0.214 W/m k by EG, and the enhancement of thermal conductivity was as high as 10.5.(3) After oxidization with the mixed acid of H2SO4 and HNO3, carbon nanotubes (CNTs) were further grafted by y-(2,3-epoxypropoxy)propyItrimethoxysilane. Then, CNTs, oxidized CNTs and grafted CNTs were respectively dispersed into palmitic acid (PA) matrix to prepare phase change composites at a mass ratio of 1/100. According to the comparative analysis results of the FT-IR spectra, morphology, dispersion and latent heat of the three composites, it is discovered that the composite with grafted CNTs showed the greatest dispersion and tube-matrix adhesion, and thus gained the highest latent heat. It is worth mentioning that the composite with grafted CNTs demonstrated a more favorable latent heat and a 34.1% enhancement of thermal conductivity in comparison with PA. Moreover, the composite with grafted CNTs kept homogeneous after 100 times of melting and freezing, and the retention rate of latent heat can be as high as 98.5%.(4) Regulating the interaction between carbonaceous materials and palmitic acid result the regulating of latent heat of composites with the optimum recipe of the above three chapters. And concluding that the palmitic acid/grafted carbon nanotubes with high latent heat, thermal stability and enhanced thermal conductivity was deem to be the optimum by comprehensively compared.