Study On The Synthesis Of Composite Carbon Nanotubes Nanofluids And The Mechanism Of Enhancing The Suspension Stability And The Thermal Conductivity

The carbon nanotubes have the properties of light weight and high thermal conductivity, and the nanofluid made with them was a new kind of strengthening heat transfer fluid, which have important potential applications in materials, mechanical, electronic, and so on. Nowadays, one of the major bottlenecks of hindering the carbon nanotubes nanofluid engineering applications is that the compatibility between carbon nanotubes and base fluid is poor, and it is difficult to disperse into homogeneous suspension, which will make the carbon nanotubes reunite, precipitate and clog the pipeline. Therefore, it is of great importance to study on the mechanism of enhancing the stability and the thermal conductivity of carbon nanotubes nanofluid.In order to solve the problems of incompatibility between the carbon nanotubes and the base fluid ethylene glycol, and poor suspension stability of the carbon nanotubes nanofluid, we developed a novel method of synthetizing composite carbon nanotubes by in-situ grafting polyethylene glycol(PEG) on the surface of carbon nanotubes. Firstly, carboxyl-functionalized carbon nanotubes(f-CNTs) were synthesized with carbon nanotubes and a mixed acid composed of nitric and sulfuric acid; then the composite carbon nanotubes(CCNTs) was synthesized via esterification reaction between the as-synthesized f-CNTs and polyethylene glycol. The surface molecular structure, surface morphology, crystal structure, specific surface area and element analysis of the CCNTs, and the polyethylene glycol content grafted on the surface of the CCNTs were characterized by Fourier Transform Infrared Spectroscopy(FTIR), Field Emission Scanning Electron Microscope(SEM), Transmission Electron Microscopy(TEM), X-Ray Diffraction(XRD), Specific Surface Area test(BET), Elemental Analysis and Thermogravimetric Analysis(TG), respectively. The results showed that the PEG has successfully grafted on the surface of CNTs.The CCNTs/EG-based NFs was prepared by suspending CCNTs in ethylene glycol by means of ultrasonic mixing. The dispersibility of the CCNTs in the EG base fluid was characterized via the Fluorescence Microscope. The rheological property was characterized by Rotational Rheometer. The suspension stability and thermal conductivity of the CCNTs/EG-based NFs were characterized by Thermal Properties Analyzer. The results show that the suspension stability and the thermal conductivity of the CCNTs/EG-based nanofluids was greatly improved by surface modification of the carbon nanotubes; and the thermal conductivity increased with the increase of the volume fraction and temperature, while the viscosity decreased with the increase of the temperature. Through analyzing the mechanism of enhancing the suspension stability and the thermal conductivity, a conclusion has been obtained that the most important mechanism would be that the polyethylene glycol grafted on the surface of carbon nanotubes has the same molecular structure with the base fluid ethylene glycol, and the polyethylene glycol grafted on the surface of carbon nanotube will be compatible with ethylene glycol, and the molecular branched chain can insert into the ethylene glycol, which will improve the dispersion stability of carbon nanotubes, and reduce agglomeration and winding of carbon nanotubes. Thereby, the Brownian motion of carbon nanotubes in ethylene glycol will become faster and so that the thermal conductivity will be increased.