The Research On Regulation On The Morphology Of The Anodic Alumina And Its Related Properties

Anodic alumina has a series of superior performance,which can meet a wide range of needs,known as a universal protective film of aluminum.Anodic alumina with different properties can be obtained by controlling different morphologies of alumina films.Therefore,it has become a focus of anodic oxidation technology to improve and broaden the range of applications of anodic alumina.In this paper,a series of alumina films with different morphologies were obtained by anodic oxidation method by changing the process parameters such as the electrolyte composition,oxidation voltage,oxidation time and reaction temperature.The microstructure and the morphology of anodic alumina were detected by X-ray diffracometry（XRD）and scanning electron microscopy（SEM）,respectively.The resistivity property and superhydrophilic property of anodic alumina films were investigated by insulation performance tester and dynamic contact meter,respectively.The main research work and results are as follows:（1）The effect of the addition of ethylene glycol on the surface morphology and the dielectric properties of the anodic alumina were systematically studied.The results show that the surface morphology of alumina film can be effectively controlled by the addition of ethylene glycol,and the dense anodic alumina film can be obtained by this way.With the increase of the content of ethylene glycol,the pore size and the porosity of anodic alumina film decreased firstly and then increased.When the content of ethylene glycol is 40 vol%,the density of anodic alumina film is the highest,and the pore density of alumina film reduce to 6.5 × 10⁹ cm^-2 and the porosity is only 1.0%.（2）The insulating properties of the anodic alumina film in the oxalic acid-ethylene glycol electrolyte system were tested.The results show that the insulation performance of the compact alumina film is greatly improved.Its breakdown strength reached 44.8V/μm,which almost increased by 84.3%.Meanwhile,its resistivity reach to 5.4×10¹³ Ω·cm,with an order of magnitude improvement compared with conventional pure aqueous solution.（3）The effects of experimental parameters such as electrolyte concentration,the anodizing time,the electrolyte temperature and the anodizing voltage on the surface morphology of alumina films were studied in pyrophosphoric acid.When the concentration of pyrophosphoric acid is 70 wt.%,a new structure of alumina nanofibers can be obtained.Prolonging the anodizing time is beneficial to obtain alumina nanofibers at high solution concentration.When the reaction temperature is 288-303 K and the reaction voltage is 30-50 V,the oxidation energy is moderate,which can obtain the alumina nanofibers with better morphology.Combined with the experimental results,the formation mechanism of alumina nanofibers was analyzed.The unique hexagonal structure of anodic alumina as well as the preferential chemical dissolution between the porous anodic alumina and the anodic alumina nanotips are the two main reason for the formation of alumina nanofibers.（4）The contact angle test show that the morphology of alumina has a great influence on the surface superhydrophilicity.Measurement of the water contact angle（WCA）reveal that the WCA of the electropolished aluminum surface is 39.5°,and the WCA of porous alumina surface have a range from 16.5° to 26.6°,which show a weak hydrophilic property.However,the nanofibers-covered aluminum surface exhibits superhydrophilic property,with a near-zero water contact angle,showing a great superhydrophilic property.