Fabrication Of Amorphous Carbon Film Combined Hierarchical Micro/Nano Structure Heat Transfer Surface And Investigation Of Anti-Fouling Property

There will often be a layer of fouling crystal adheres on the heat transfer surface during its service time,which will extends the fouling induction period and increases the heat resistance of the heat transfer surface,resulting in the reduction of heat transfer efficiency of heat exchange equipment,the increase in equipment energy consumption and the shortening of service life of heat exchange equipment.Obviously,the research on the enhancement of anti-fouling property of heat transfer surface possesses important scientific significance and application value.In this thesis,a two-step method combining liquid phase pulse discharge technology and magnetron sputtering technology is proposed to prepare a composite layer of amorphous carbon film with micro-nano structure(HMNS)on the heat exchange surface.The surface characteristics,including morphology,surface roughness(Ra and Rz),wettability(contact angle,CA)and anticorrosion property of HMNS composite layer surface are investigated,especially the structure-activity relationship between surface characteristic and anti-fouling property and the application of HMNS surface in the enhancement of heat transfer by pool boiling is also investigated.Based on the principle and characteristic of liquid phase pulse discharge technology,electrical discharge machining(EDM)is proposed to prepare HMNS on copper metal surface.The characteristic parameters,including surface roughness and contact angle of the as-prepared HMNS surface are characterized by optical microscope(OM),high resolution scanning electron microscope(HRSEM),scanning electron microscope(SEM),atom force microscope(AFM)and Energy-dispersive X-ray spectroscopy(EDS).The influence law of pulse discharge parameters on surface roughness and wettability is studied by orthogonal experimental design.The results show that the surface characteristics of HMNS can be controlled by the modification of EDM parameters.The sequence of factors affecting Ra is current,pulse width,gap voltage and ratio factor.The sequence of factors affecting Rz and CA is current,pulse width,ratio factor and gap voltage.Copper based HMNS surface is hydrophobic and its static contact angle is increases with the increase of pulse width.In the experiment of this thesis,the CA can reach 144.7°±2.1° and the CA hysteresis is 8.46°±3.3° to 14.1°±1.2°.Micro-nanostructured pores in the surface of the copper-based HMNS can store air and form "air cushion"effect,which is the key to increasing surface hydrophobicity.The result of the fsl,which is the area fraction for the liquid-solid interface of the copper-based HMNS surface and water droplets,shows that the water droplets and the surface of the micro-nano structure have only a contact area of 14.05%to 25.10%,and the other contact areas are the contact of water droplets and "air cushion".Therefore,water droplets have the tendency to agglomerate and difficult to spread on the surface of the copper-based HMNS,improving the contact angle of copper-based HMNS surface.The wettability of amorphous carbon film is relatives to its sp2 content.The sp2 content can be controlled by the modification of bias voltage of magnetron sputtering,resulting in the adjustment of the CA of amorphous carbon film.The results showthat the surface of amorphous carbon film prepared by magnetron sputtering is composed of nano-sized spherical amorphous carbon clusters.With the increase of bias voltage,the average size of the spherical amorphous carbon clusters is increases,the average distance between the clusters is decrease,leading to the decrease of the number of holes in the amorphous carbon film and the sp2 content of amorphous carbon film is increases first and then decreases within the range of 59.02%- 70.50%.Moreover,with the increase of ID/IG,sp2/sp3 and sp2 content of amorphous carbon film,the CA of amorphous carbon film is increases,while the surface energy of amorphous carbon film is decreases.According to the Cassie-Baxter wetting principle,the hydrophobicity of amorphous carbon film is enhanced by the synergistic effect of the holes in the amorphous carbon film act as storage air to hinder the droplets from spreading on the surface and the low surface energy of amorphous carbon film.The CA and surface energy of hydrophobic amorphous carbon film under different bias voltage are as follow:-100V(133.16°±0.45°,4.57 × 0.29 mJ/m2),-200V(119.57° ± 1.17°,11.73 × 1.89 mJ/m2)and-300V(121.28°± 0.20°,10.57 × 1.53 mJ/m2).The anti-fouling and pool boiling heat transfer property of copper-based HMNS surface are systematically studied by self-made heat transfer surface coating heat transfer performance test system.The influences of hydrophobicity,surface roughness and anticorrosion property on the anti-fouling property of copper-based HMNS are investigated.The results show that the copper-based HMNS has the longer fouling induction period and lower max fouling resistance(Rf max)in the lower heat flux condition and in the higher heat flux,the fouling induction period of copper-based HMNS disappears,but its Rf max is still lower than that of copper polished surface.There are three reasons for this result.Firstly,hydrophobicity of copper-based HMNS reduces the adhesion of fouling crystal on the heat transfer surface and the adhered fouling crystal is loose.Secondly,the anticorrosion property of copper-based HMNS decreases the concentration of Cu2+ion in the fouling solution,which decreases the deposition of complicate carbonate fouling crystal.Finally,,the copper-based HMNS has an effect similar to porous surface,which enhances the activity of bubble and increases the detach frequency of boiling bubble.As a result,suffering from the "wiping effect"caused by the detachment of boiling bubble,the adhered fouling crystals also fall off from the heat transfer surface.The synergistic effect of all above factors endows the copper-based HMNS with remarkable anti-fouling property.In addition,the adhered fouling crystals on the copper-based HMNS can be cleaned by the ultra-sonic cleaning and the CA of ultrasonic cleaned just.slightly decreases when compared to the original surface.For the pool boiling heat transfer property,the area of boiling bubble nucleation site is increase by the HMNS,which enhances the bubble activity on the heat transfer surface,resulting in the increase of the critical heat flux(CHF)and heat transfer coefficient(HTC)of copper-based HMNS.On one hand,the higher surface roughness of copper-based HMNS increases the surface area of heat transfer,resulting in the increase of CHF.On the other hand,the hydrophobicity of copper-based HMNS increases the detach frequency of boiling bubble on the heat transfer surface,leading to the enhancement of HTC.A composite layer of HMNS amorphous carbon film(MES)is fabricated by a two-step method combining liquid phase pulse discharge technology and magnetron sputtering technology.The macro and micro surface morphologies of MES,structure and chemical valence of element of amorphous carbon film are detected by digital camera,HRSEM,SEM,EDS,XPS,XRD and Raman,respectively.The anticorrosion,hydrophobicity and oleophobicity,and anti-fouling properties of MES is respectively evaluated by electrochemical working station and atomic absorption spectrophotometer,contact angle measurement and self-made heat transfer surface coating heat transfer performance test system.The results show that the MES displays more nano structures than that of copper-based HMNS.The sp2/sp3 ratio of amorphous carbon film in MES is 1.98,indicating that its degree of graphitization is high and surface energy is low.Thus,MES shows superior hydrophobicity and oleophobicity with CA of 146.4°±3.4°(Water)and CA of 150.2°±3.6°(Glycerin).Meanwhile,the MES shows better anticorrosion property with higher impedance value than that of copper-based HMNS and polished copper surface.These results indicate MES has a stronger barrier effect on the corrosive medium and reduces the effective contact area between the corrosive medium and the composite HMNS surface,improving the corrosion resistance of the heat exchange surface of the sample.Basic on the co-effect of above factors,MES shows superior anti-fouling property under the condition of the heat flux of 66 kW/m2.When compared with copper-based HMNS and polished copper surface,MES displays lower fouling resistance and longer fouling induction period,which is 3.8 times longer than that of copper-based HMNS.The combination of liquid phase pulse discharge technology and magnetron sputtering technology can combine the nanoscale features on the micro/nano scale structure prepared by liquid phase pulse discharge machining,enhancing the composite degree of multi-scale features on HMNS.Besides,it is convenient to construct a material system of the micro-nano structure surface and regulate the surface energy to a low energy state.Based on the above characteristics,the hydrophobic,anti-corrosion and anti-fouling properties of HMNS surface are enhanced significantly.