交联碳纳米管定型Na_(2)SO_(4)·10H_(2)O基相变储能材料制备及性能表征

Preparation and Performance Characterization of Cross-Linked Carbon Nanotubes Shaped Na_(2)SO_(4)·10H_(2)O Based Composite Phase Change Energy Storage Materials

针对于Na_(2)SO_(4)·10H_(2)O相变储能材料的过冷、相分层及易泄露等问题,通过Friedel-Crafts烷基化和高温碳化的方法制备交联碳纳米管(CL-CNTs),采用熔融法制备了交联碳纳米管定型Na_(2)SO_(4)·10H_(2)O基相变材料并研究其性能。结果表明:大多数交联的CL-CNTs呈现管状结构,管状结构表面生长有少量碳球,交联碳纳米管上具有羟基和苯环等基团,成功制备出亲水性交联碳纳米管。分别添加质量分数为1%到5%的碳纳米管制备定型相变储能材料,当样品质量分数达到4%时(标记为样品PCMs-4),样品呈现最佳定型状态。对样品PCMs-0和样品PCMs-4的导热系数和相变潜热进行了分析,其中样品PCMs-4的导热系数显著提升,在20℃时导热系数最高值为1.008 W·m^(-1)·K^(-1)。芒硝相变储能材料相变循环500次后,样品PCMs-4较之样品PCMs-0表现良好的循环热稳定性,PCMs-0的相变循环前后的熔化焓损失率为18%,结晶焓损失率为25%,样品PCMs-4的相变循环前后的熔化焓损失率为2%,结晶焓损失率为2.3%,综合分析可知样品PCMs-4相变储能材料有效减少相分层现象,提高芒硝相变材料的循环稳定性。

Introduction Mirabilite as a phase change energy storage material becomes a research hotspot because of its high latent heat,low material cost,and high thermal conductivity.However,the problems of supercooling,phase delamination,and easy leakage in the phase transition cycle seriously restrict its application.Some work indicate that stereotype-phase change materials can effectively solve these problems.In this paper,crosslinked carbon nanotubes(CL-CNTs)were prepared via the Friedel-Craft alkylation and high-temperature carbonization.The CL-CNTs shaped Na_(2)SO_(4)·10H_(2)O based phase change materials were prepared via melting,and their properties were investigated.Methods Pyrene(1×10~(-3)mol),1,2-dichloroethane(20 mL),and FeCl_(3)(3×10~(-3)mol)were added into 100 mL beakers.After FeCl_(3)was dissolved,methyl acetal(3×10~(-3)mol)was added and stirred magnetically at room temperature for 5 h.At the end of the reaction,the solution was filtered,and the solid obtained was alternately rinsed with methanol,distilled water,methylene chloride,and acetone.The cleaned solid material was extracted with methanol Soxhlet for 48 h and then dried in vacuum for 7 h.The yellow powder was heated to 650℃in a tube furnace with a nitrogen atmosphere for 4 h.Finally,it was cooled to room temperature and carbonized to obtain a black powder.4.0%NaCl,1.0%borax,and 0.1%sodium metaphosphate were added into a mixture of Na_(2)SO_(4)·10H_(2)O and Na_(2)CO_(3)·10H_(2)O(9:1)to prepare a phase change energy storage material of mirabilite.Afterwards,CL-CNTs were added into the phase change material with a mass fraction of 0,1%,2%,3%,4%,and 5%,respectively,and the phase change material was prepared by a melting intercalation method.These samples were ultrasonically treated for 24 h in the molten state and then stirred in a mixer for 2 h,which were labeled as PCMs-0,PCMs-1,PCMs-2,PCMs-3,PCMs-4,and PCMs-5,respectively.Results and discussion Most CL-CNTs prepared show a tubular structure,and a few carbon spheres grow on the carbon nanotubes.Hydrophilic CL-CNTs were prepared with hydroxyl and benzene ring groups on the CL-CNTs.Na_(2)SO_(4)·10H_(2)O shaped phase change materials were prepared via melting intercalation at different mass fractions of carbon nanotubes(i.e.,1%,2%,3%,4%,and 5%).The samples at>4%CL-CNTs show a superior stereotyped state.According to the analysis by SEM,a uniform mixture of PCMs-4 phase change material and the crystalline particles of the mirabilite phase change material exist around the carbon nanotubes,indicating that CL-CNTs shaped Na_(2)SO_(4)·10H_(2)O phase change material can be prepared.The thermal conductivity of PCMs-4 is greater than that of PCMs-0,and the thermal conductivity increases with the increase of temperature at 0-20℃and at 30-50℃.At 20℃,the thermal conductivity has the maximum value.At 20-30℃,the thermal conductivity of PCMs-0 and PCMs-4 decreases due to the heat absorption during the solid-liquid phase change of hydrated salt.Based on the DSC data of PCMs-0 and PCMs-4 before and after 500phase transition cycles,PCMs-4 has better cyclic stability than PCMs-0.The enthalpy loss rate of melting and crystallization before and after phase transition cycles of PCMs-0 is 18%and 25%,respectively,indicating that a phase stratification occurs as phase transition cycle increases.The latent heat release of phase change decreases,and the Na_(2)SO_(4)·10H_(2)O-based hydrate salt gradually fails.The melting enthalpy loss rate of sample PCMs-4 before and after the phase transformation cycle is 2%,and the crystallization enthalpy loss rate is 2.3%,indicating that,the potential heat of phase transformation is not significantly reduced as the number of phase transformation cycle is increased.It is indicated that adding CL-CNTs to PCMs-4 can effectively reduce the phase stratification and improve the cycle stability of mirabilite phase change materials.Conclusions The shaped Na_(2)SO_(4)·10H_(2)O based phase change materials were prepared,thus effectively solving the problems of supercooling and phase stratification of materials.The sample PCMs-4 had a good shape.The thermal conductivity reached a maximum value of 1.008 W·m^(-1)·K^(-1)at 20℃.There was still a high melting latent heat of 230.1 J/g and solidification latent heat of205.2 J/g after the phase transformation cycle,indicating that the latent heat of phase transformation was not reduced as the number of phase transformation cycle was increased.