Effect Of Surfactant On Thermal Conductivity And Solidification Properties Of Nanoparticle Suspension

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.