【作者】 贾莉斯；

【导师】 彭岚；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 过冷度小或无过冷，高导热系数的纳米悬浮液用作相变储能材料是提高储能系统效率，降低能耗的有效方法。纳米悬浮液的性质在循环冷却和加热过程中保持稳定是将其应用于实际必须具备的前提条件。分散剂的使用是制备出分散稳定纳米悬浮液的重要手段。借助分散剂的作用，较大尺寸的纳米团聚体可在基液中分裂成众多较小尺寸的团聚体或纳米颗粒，这些团聚体或颗粒上的分散剂吸附层能对其表面润湿性进行修饰，进而影响到纳米悬浮液的凝固成核，分散剂吸附层本身的导热系数也将对纳米悬浮液的导热系数产生影响。基于此，本文以水基、石蜡基和月桂酸基纳米悬浮液为研究对象，重点研究了分散剂对这三种纳米悬浮液的导热和凝固结晶生长特性的影响及其作用机制，获得了增强纳米悬浮液热循环稳定性的有效控制手段和条件。主要研究工作及结果如下：①采用超声振动与分散剂相结合的方法制备TiO2-水、Graphite-水、Al-石蜡和TiO2-月桂酸纳米悬浮液，采用Zeta电位及粒径分析仪测试纳米悬浮液的Zeta电位和粒径分布、电子透射电镜和场发射扫描电子显微镜测试纳米团聚体的形貌及其分布，研究分散剂种类、浓度和分散剂/纳米颗粒浓度比对各纳米悬浮液分散稳定性的影响。结果表明：阴离子型分散剂增强水基纳米悬浮液分散稳定性的作用最强，其中十二烷基硫酸钠SDS对TiO2-水纳米悬浮液的分散稳定效果最佳，羧甲基纤维素钠CMC对Graphite-水纳米悬浮液的分散稳定效果最佳。SDS/TiO2浓度比是影响TiO2-水纳米悬浮液分散稳定性的重要因素，最佳SDS/TiO2浓度比约为1:1；对于有机TiO2-月桂酸和Al-石蜡纳米悬浮液，具有最佳分散稳定作用的分散剂分别为阴离子型分散剂SDS和阳离子型分散剂十六烷基三甲基溴化铵CTAB。②采用最小热阻力法则和比等效导热系数相等法则建立了考虑分散剂作用的纳米悬浮液静态导热系数模型，该模型同时考虑了纳米颗粒团聚和分散剂吸附层的影响。采用闪光法导热仪测量TiO2-月桂酸纳米悬浮液的固相导热系数，X射线光电子能谱分析仪测量TiO2纳米团聚体表面上分散剂SDS吸附层的厚度，场发射扫描电子显微镜测量TiO2纳米团聚体的大小形貌及其分布，研究分散剂SDS浓度对TiO2-月桂酸纳米悬浮液导热系数的影响及其机理。结果表明：分散剂SDS的加入降低了TiO2-月桂酸纳米悬浮液的导热系数，降低程度与分散剂SDS浓度密切相关。分散剂SDS浓度通过改变TiO2纳米团聚体大小和分散剂SDS吸附层厚度，对TiO2-月桂酸纳米悬浮液的导热系数起作用。当分散剂SDS浓度与TiO2纳米颗粒浓度相当时，分散剂SDS对TiO2-月桂酸纳米悬浮液导热系数的削弱程度达到最大。与现有Maxwell模型、Yu-Choi模型、Xue模型、Xie et al.模型和Leong et al.模型相比，新建模型对TiO2-月桂酸纳米悬浮液固相导热系数的理论计算结果可以与实测结果更好的吻合，预测偏差在5%以内。基于上述结果，采用Hotdisk导热仪分别对TiO2-水纳米悬浮液的固相和液相导热系数进行了测量，探讨纳米团聚体自身对纳米悬浮液导热系数的强化机理。结果表明：TiO2纳米团聚体的热传导和布朗运动均对TiO2-水纳米悬浮液的导热系数有重要的强化作用，随着纳米颗粒浓度的增大，TiO2-水纳米悬浮液导热系数中的静态部分所占权重逐渐增大。③采用差示扫描量热仪DSC测试TiO2-水纳米悬浮液的凝固过程、采用表面张力法测量分散剂SDS在TiO2纳米团聚体表面的吸附量，研究分散剂浓度和冷却速率对TiO2-水纳米悬浮液凝固的相变温度、时间和潜热的影响及其作用机制。结果表明：大冷却速率（≥5℃/min）下，冷却速率是控制TiO2-水纳米悬浮液凝固成核的主导因素，此时TiO2-水纳米悬浮液凝固的相变温度低、相变时间短、相变潜热少；小冷却速率（＜5℃/min）下，分散剂控制的TiO2纳米团聚体表面成核在TiO2-水纳米悬浮液的凝固成核中占主导，此时TiO2-水纳米悬浮液凝固的相变温度高、相变时间长、相变潜热多。分散剂SDS浓度通过调节分散剂在TiO2纳米团聚体表面的吸附量，对TiO2-水纳米悬浮液的凝固成核产生影响。当分散剂SDS浓度与TiO2纳米颗粒浓度相当时，分散剂SDS的吸附达到饱和，TiO2-水纳米悬浮液的凝固成核温度达到最高。差示扫描量热法研究TiO2-月桂酸和Al-石蜡纳米悬浮液固-液相变特性的结果显示：TiO2（Al）纳米颗粒的加入降低了月桂酸（石蜡）的相变潜热，分散剂SDBS（CTAB）的使用对此没有明显的改善作用。④Graphite-水和Al-石蜡纳米悬浮液在低温槽内的宏观凝固实验显示，在纳米悬浮液的凝固过程中，纳米颗粒会被凝固界面排斥而聚集在一起，再次溶解后出现沉降，造成纳米悬浮液失效；分散剂对纳米悬浮液在凝固过程中的分散稳定性的改善作用不大。施加超声振动可以得到Al纳米团聚体均匀分布的Al-石蜡纳米悬浮液凝固体，但凝固体中存在的气泡导致其导热系数明显降低。施加磁场可以改善Graphite纳米团聚体在其纳米悬浮液凝固体中的均匀分布。磁场的这种作用与分散剂在Graphite纳米团聚体表面的吸附密切相关。与分散剂SDS相比，分散剂CMC在Graphite纳米团聚体表面的吸附能力更强，Graphite团聚体表面带电荷数更多，磁场增强Graphite-CMC-水纳米悬浮液凝固稳定性的作用更明显。更多还原

【Abstract】 Nanoparticles suspensions with low or none supercooling degree and high thermalconductivity as the phase change materials is good for increasing system’s energystoring efficiency and reducing energy consumption. Keeping nanoparticles suspensionsstable in the process of cyclic heating and cooling is the precondition for applying it tothe actual application. Adding surfactant is an important method for preparing dispersedstable nanoparticles suspensions. Larger-size nanoparticles may be divided into lots ofsmaller-size nanoparticles and agglomerates by the effect of surfactant. The surfactantadsorption layer on the smaller-size nanoparticles and agglomerates can modify surfacewettability, and then influence the solidification nucleation in nanoparticles suspensions.The thermal conductivity of surfactant adsorption layer may also influence the thermalconductivity of nanoparticles suspensions. Therefore, we take water-based,paraffin-based and lauric acid-based nanoparticles suspensions as the research objects,particularly focus on the influence of surfactants on the thermal conductivity andsolidification crystal growth characteristic of the three nanoparticles suspensions. Andthen effective controlling method and condition of enhancing nanoparticles suspensions’thermal cycle stability have been obtained.The main research work and the results are as follows:①TiO2-water, Graphite-water, Al-paraffin and TiO2-lauric acid nanoparticlessuspensions are prepared by the method of ultrasonic vibration and adding surfactant.The zeta potential and size distribution of nanoparticles suspensions are measured byzeta potential and particle size analyzer. The morphology and distribution ofnanoparticles agglomerates are measured by transmission electron microscope and fieldemission scanning electron microscopy. The influences of kinds of surfactant,concentration and concentration ratio of nanoparticles suspensions to the stability ofnanoparticles suspensions are investigated. The results show that anionic surfactant hasthe best effect on enhancing the stability of nanoparticles suspensions; Sodium DodecylSulfonate (SDS) has the best effect on enhancing the stability of TiO2-H2Onanoparticles suspensions and Sodium carboxymethylcellulose (CMC) has the besteffect on enhancing the stability of Graphite-water nanoparticles suspensions. Theconcentration ratio of SDS and TiO2is an important factor of influence the stability ofTiO2-H2O nanoparticles suspensions. The best concentration ratio of SDS and TiO2is 1:1. For Al-paraffin and TiO2-lauric acid nanoparticles suspensions, the best surfactantare cationic surfactant Cetyltrimethyl Ammonium Bromide (CTAB) and anionicsurfactant SDS.②The nanoparticles suspensions static thermal conductivity model whichconsidering the effect of surfactant is built by the methods of the minimum thermalresistance law and criteria of equivalent specific thermal conductivity. The modelinvolves both the effect of the nanoparticles’ aggregation and surfactant adsorption layer.The thermal conductivity of solid-state TiO2-lauric acid nanoparticles suspensions ismeasured by flash method conductometer. The thickness of SDS adsorption layer on thesurface of TiO2agglomerates is measured by X-ray photoelectron energy spectrumanalyzer. The size, morphology and distribution of TiO2agglomerates are measured byfield emission scanning electron microscopy. The concentration influence of SDS to thethermal conductivity of TiO2-lauric acid nanoparticles suspensions and effectmechanism were investigated. The results show the thermal conductivity of TiO2-lauricacid nanoparticles suspensions decline because of adding SDS, and the reduction degreeis closely related to the concentration of SDS. By changing the size of TiO2agglomerates and the thickness of SDS adsorption layer, the concentration of SDSimpacted on the thermal conductivity of TiO2-lauric acid nanoparticles suspensions.When the concentration of SDS and TiO2is comparable, SDS has the most significanteffect on reducing the thermal conductivity of TiO2-lauric acid nanoparticlessuspensions. Comparing to previous models such as Maxwell, Yu-Choi, Xue, Xie et al.,and Long et al, the new nanoparticles suspensions static thermal conductivity modelcould fit the experiment results of TiO2-H2O nanoparticles suspensions thermalconductivity more, within5%deviation. Based on above results, the thermalconductivity of solid-state TiO2-water nanoparticles suspensions is measured byhot-disk conductometer. Strengthening mechanism of thermal conductivity coefficientby nanoparticles aggregation in nanoparticles suspensions were discussed. The resultsshow that the heat conduction and Brownian motion play important role in enhancing hethermal conductivity of TiO2-water nanoparticles suspensions, and the static portion ofthermal conductivity is enhanced with the increasing concentration of nanoparticles.③The solidification process of TiO2-water nanoparticles suspensions is measuredby differential scanning calorimeter (DSC), and the SDS adsorbing capacity on thesurface of TiO2agglomerates is measured by surface tension method to investigate theinfluence of concentration and cooling rate of surfactant on the solidification phase-transition temperature, time and latent heat and effect mechanism of TiO2-H2Onanoparticles suspensions. Results show that the cooling rate is dominant factor ofsolidification nucleation of TiO2-H2O nanoparticle suspensions as a result of lowphase-transition temperature, short time and less latent heat at high cooling rate (≥5℃/min), while the surface nucleation is major factor of solidification nucleation of TiO2-H2O nanoparticle suspensions as a result of high phase-transition temperature, long timeand more latent heat at low cooling rate (<5℃/min). The SDS concentration plays arole in the solidification nucleation of TiO2-water nanoparticle suspensions throughadjusting the SDS adsorbing capacity on the surface of TiO2agglomerates. When theconcentration of SDS and TiO2are the same, SDS adsorption reaches saturate whichleads a peak nucleation temperature.④The macroscopic solidification experiments of Graphite-water and Al-paraffinnanoparticle suspensions in cryostat prove that nanoparticles will aggregate because ofthe rejection of solid-liquid interface and then the precipitate appear leading the failureof nanoparticle suspensions when it melts in the process of solidification, whichillustrates that the surfactant has little effect on enhancing the stability of nanoparticlessuspensions in the process of solidification. Al-paraffin nanoparticle suspensions withhomogeneous distribution can be obtained through ultrasonic vibration, but the thermalconductivity decreases obviously since the bubbles exist in the process of solidification.Graphite agglomerates would disperse in Graphite-water nanoparticle suspensionshomogeneously in a magnetic field. The effect of magnetic field is closely related to theadsorbing of surfactant on the surface of Graphite agglomerates. Compared with SDS,CMC has a stronger ability of adsorbing and more charges on the on the surface ofGraphite agglomerates, and the magnetic field enhances the stability of nanoparticlesuspensions more obviously.更多还原