【作者】 汪超；

【导师】 任忠鸣； 钟云波；

【作者基本信息】 上海大学 ， 钢铁冶金， 2011， 博士

【摘要】 磁场下电沉积是近些年来快速发展的一种电沉积技术,它利用磁场与电场的交互作用,对金属电沉积过程、镀层结构形貌及性能产生显著影响,引起了国内外研究者的极大兴趣。然而大多数研究集中在磁场对单质金属或者合金的影响,有关磁场对复合电沉积的影响的研究相对较少。纳米复合镀层具有优良的耐腐蚀性、耐摩擦性以及高硬度等物理和化学性能,近些年得到了很多研究者的注意。但是到目前为止如何有效提高镀层中的颗粒含量以及控制颗粒的分布等问题仍未解决。研究者希望借助磁场具有无接触,在电沉积过程中能够产生洛伦兹力、磁力等优势,来提高电沉积的传质过程和电子转移过程,利用MHD流动改变镀层结构和颗粒的分布,因此磁场对纳米复合电沉积的影响亟待深入研究。本文以Ni-纳米Al₂O₃电沉积体系为研究对象,开展了水平磁场和竖直磁场中复合电沉积的研究。本文采用水平磁场和竖直磁场,通过改变磁感应强度、电流密度和磁场与电场相对方向（垂直磁场和平行磁场）,开展了以下几方面的基础研究:1.采用沉降比的方法研究了制备复合镀液的最佳方法。考察了分散方式、分散时间和分散剂含量对复合镀液稳定性的影响,得出最佳的Ni-纳米Al₂O₃复合镀液制备条件为:分散剂含量为1g·L^-1,采用电磁搅拌和超声波分散相结合的形式,超声波分散1小时。2.在成功制备了复合镀液之后,作者研究了无磁场条件下电流密度对纯镍镀层和复合镀层的影响。研究发现纯镍镀层呈金字塔状生长,晶粒粗大。随着电流密度增大,纯镍镀层从（111）择优生长晶面逐渐向（100）择优生长晶面转变。而加入纳米氧化铝颗粒以后,晶体呈包状生长,每个晶包都由细小的纳米晶组成。镀层全部呈（111）择优取向,电流密度对复合镀层的择优取向没有影响。3.为了考察磁场对复合电沉积的影响,作者首先在水平磁场中考察了磁感应强度、磁场方向和电流密度对复合镀层的影响。研究发现当磁场与电流相互垂直时,氧化铝颗粒在镀层中弥散分布,并且当产生洛伦兹力方向向上,颗粒含量随着磁感应强度的增大而增大,反之则降低。并且在0.5T垂直磁场中,颗粒含量在电流为2A·dm^-2时出现一个最大值。在平行磁场中,颗粒不再弥散分布,趋于在晶包边界沉积,其含量随着磁感应强度的升高变化不大,并且小于在垂直磁场中得到的颗粒含量。研究还发现在磁场中制备的复合镀层的择优取向没有改变,（111）是其择优取向。在垂直磁场中复合镀层的微硬度随着磁感应强度的增加而增大,主要归因于镀层中颗粒的含量增多和晶粒的细化。研究表明复合镀层的硬度明显比纯镍镀层的硬度高。4.鉴于水平磁场中磁感应强度的限制（小于1T）,有必要考察强磁场（大于1T）中复合镀层受磁场的影响规律。利用竖直强磁场,作者考察了磁感应强度和电流密度在垂直磁场和平行磁场两种条件下对复合镀层结构、形貌和成分的影响规律。当磁场与电流垂直时,颗粒在复合镀层中弥散分布,其含量随着磁感应强度的增加而增大,磁场为8T时达到极大值4.6wt%左右,随后降低。在10T垂直磁场中改变电流密度,镀层中颗粒含量在2A·dm^-2时出现一个最大值。镀层微观结构并没有受磁感应强度和电流密度的影响。当磁场与电流平行时,氧化铝颗粒在镀层中明显呈蜂窝状分布,且其含量随着磁感应强度的增大而增加,且在6T时达到极大值约24wt%。10T平行磁场镀层中颗粒含量随着电流密度的升高而升高,且在2A·dm^-2时出现最大值。分析表明镀层中颗粒含量受MHD效应强度的影响。MHD流动过小或者过大都不利于纳米颗粒在镀层中沉积。通过分析复合镀层在平行磁场中的生长过程,作者提出了平行磁场中颗粒规律分布的形成机理:在微观范围内,镀层在阴极上沉积一定时间以后,很多晶粒组成的晶包是凹凸不平的,电流经过晶包前端时,由于晶包和周围溶液电导率的差别,电流线就会弯曲,从而形成垂直磁场方向的电流分量,该电流分量与磁场作用产生洛伦兹力,在晶包前端形成微观流动,这种流动驱动带电的氧化铝颗粒向晶包边界移动并沉积。这种机理打破了日本人认为纳米颗粒不能形成蜂窝状分布的论断。5.为了研究磁场对传质及电子转移过程的影响,作者用电化学方法首先研究了无磁场时分散剂和纳米颗粒对复合电沉积过程的影响。循环伏安法研究发现离子并不是直接放电生成原子,而是首先生成某种中间体,然后再放电生成原子或者气体。计时电流法研究发现在瓦特镀液中,镍倾向于三维圆锥形BFT（Bewick A., Fleischmann M. and Thirsk, H. R.）模型生长,而加入分散剂和氧化铝颗粒以后倾向于半球形的SH（Scharifker and Hill）模型生长。这与SEM观察到的结果一致。交流阻抗法研究发现阿拉伯树胶在阴极吸附,降低了阴极活化面积,增大了镍沉积过电位。而纳米氧化铝颗粒在阴极吸附或者进入镀层增加了结晶形核质点,有利于纳米晶的形成。6.在了解分散剂和颗粒浓度对复合电沉积过程的影响的基础上,作者用电化学方法又研究了水平弱磁场和竖直强磁场中磁感应强度和磁场相对方向对复合电沉积过程的影响。循环伏安法研究发现:磁场与电流相对位置不同,磁场对循环伏安曲线的峰电流和极限电流影响不同。在垂直磁场中,磁场对电沉积过程影响较大,而在平行磁场中,磁场对电沉积过程影响较小。线性扫描法研究发现正垂直磁场加强了极限电流,负垂直磁场减小了极限电流,正平行磁场加强了极限电流,但是磁感应强度影响不大,反平行磁场中,极限电流随磁感应强度的增加而增加,但是当电位低于-0.98V时,极限电流反而随磁感应强度的增加而降低。交流阻抗法研究发现磁场的施加降低了电子转移阻力。由于在竖直强磁场中,一方面由于MHD流动,使得金属离子在溶液的运动轨迹发生偏转;另一方面受电磁制动力的影响,又阻碍了离子运动轨迹的偏转。与此同时MHD效应使氧化铝颗粒的传质过程加强,到达阴极的数量增加,使得双电层电容值呈现先下降后上升,电子反应电阻值先上升后下降的规律。7.本文的研究表明磁场对电沉积过程中的传质过程产生明显影响。对于竖直放置的电极,当洛伦兹力与自然对流驱动力方向相同时,磁场的施加增强了传质过程,增加了镀层中颗粒含量。当洛伦兹力与自然对流驱动力相反时,洛伦兹力引起的流动抵消了自然对流对传质过程的影响,随着磁感应强度增加,洛伦兹力增加,还原电流降低。综上所述,本文的研究表明,磁场的施加,对镍-纳米氧化铝复合电沉积过程的传输过程、电子转移过程、镀层的形核长大过程均产生了显著的影响,而在平行磁场中,在阴极表面附近区域形成的微观MHD流动,对复合镀过程的阴极过程影响显著,甚至可以控制镀层中纳米陶瓷颗粒的分布,进而改变镀层的综合性能。因此本文的研究对制备新型的金属-纳米陶瓷颗粒复合材料、功能镀层均具有重要的借鉴意义。更多还原

【Abstract】 In recent years, heavy attention has been paid to the process of Magneto-electrodeposition by scientists majored in electrochemistry, metallurgy and material due to the remarkable interaction between magnetic field and electric field. However, the former’s works mostly focused on the metal or alloy deposition in magnetic field, less attention has been devoted to the composite electro-deposition. As we know, the composite electrodeposition is an very important way to obtain metallic composite material with excellent mechanical, physical and chemic properties, however, there still remain a lot of questions to be solved, for example, how to elevate the content of nano ceramic particles or control the distribution of the nano ceramic particles are still key points for preparing nano composite deposit. Considering the lorenz force, magnetic force can be induced in the electrolyte by superimposing an static magnetic field without contact, which will be helpful to influence the mass transport process, the electron transfering process, even the nucleation and growth of the deposit, So, the electro-deposition of Ni-Al₂O₃ composite coating in horizontal magnetic field （<1Tesla） and strong vertical magnetic field (1^-12Tesla) was discussed in this paper.To study the effect of magnetic field, the horizontal magnetic field and vertical magnetic field were used, in which the process of electrodepositing Ni-nano Al₂O₃ composite was conducted with changing magnetic flux density and current density as well as the relationship between the direction of magnetic field and electric field. The main contents and conclusions in this paper were shown as followings:1. Preparing a stable composite deposition solution is the first step in this work. Sedimentation rate method was used to study the influence of dispersion type, arabic gum content and dispersion time on the sedimentation rate of alumina nano particles in the solution. According to experimental results, the best condition to prepare composite solution as following: arabic gum content 1g/L, ultrasonic disperse for one hour.2. After preparing the composite deposition solution, the author considered the effect of current density on the pure Ni film and Ni-Al₂O₃ composite film without magnetic field. The experimental results showed that the surface of pure nickel film showed pyramidal type. The preferred orientation changed from <111> to <100> with current density increased from 1 A·dm^-2 to 4 A·dm^-2. The surface of composite coating was composed of many lumps which consisted by a lot of nanosize crystals. The preferred orientation of composite coating was <111>, which was not affected by current density.3. A horizontal magnetic field was firstly used to examine the applied magnetic field on the deposition of composite coating. In this field, the magnetic flux density and current density as well as relative orientation of magnetic field with current field must be considered. The results showed that alumina particles dispersed uniform in the coating and its content increased with magnetic flux density increased if the Lorentz force was upward. The particle content also affect by applied current density, a maximum value was got when the current density increased to 2 A·dm^-2 and then decreased with the current density increased more higher. In parallel magnetic field, the alumina particles distributed different with that in perpendicular magnetic field. The alumina particles almost were deposited at the boundary of the lumps. It was also found that the preferred orientation was （111） and could not be influence by the magnetic field and current density. The microhardness experiment showed that the composite coating was harder than the pure nickel coating and the values increased with magnetic flux density increased in perpendicular magnetic field, which might due to the dispersion strengthening effect and the refined crystal by included alumina particles.4. For the magnetic flux density in horizontal magnetic field was smaller than 1T, it was necessary to carry experiments out in vertical high magnetic field which was higher than 1T. In the high vertical magnetic field, the influence of magnetic flux density and current density on the surface, micro-texture and composition of composite coating was studied. In the perpendicular magnetic field, the alumina particles dispersed in the composite coating and its content reached a maximum 4.6wt% with the magnetic flux density increased to 8T then decreased. The effect of current density on the particle content in 10T magnetic field was also taken into mind. The results showed that the particle content increased first and then decreased with current density increased, and its value reached the maximum under the current density of 2 A·dm^-2. Different results were got in the parallel magnetic field. The alumina particles dispersed in a network pattern in the film and its content increased with magnetic flux density increased and reached the maximum 24wt% in the magnetic fild of 6T. As same as results in perpendicular magnetic field, the particle content reached the maximum 22wt% under the current density of 2A·dm^-2 in 10T magnetic field. In all the condition, the preferred orientation was （111） and could not be influenced by the magnetic field and current density. Depending on the experimental results, a model was suggested after analyzing the growth process of composite coating to illuminate the network distribution of alumina particles in the film. In this model, it was considered that the surface of cathode became rougher with film grew and a lot of lumps appeared on the cathode. In front of the lumps, the faradic currents were not parallel to the magnetic field strictly, so Lorentz force could be induced. As a result, anticlockwise eddy would be generated in front of lumps and the positive charged particles in the eddy rotated along with the electrolyte. Under the influence of Lorentz force, the charged particles finally be drawn to eddy edge and then deposited on the edge of lumps. All the results showed that a suitable MHD effect was necessary to get higher particle content.5. Electrochemical methods were also used to study the effect of organic addition and alumina nano particles on the deposition process of Ni-Al₂O₃ system. The CV （cyclic voltammetry） results showed that a kind of intermediate was generated before nickel ions or hydrogen ions discharged. Chronoamperometry curve indicated that the nucleation of nickel in Watts type solution had a good agreement of BTF model, but the addition of dispersant and alumina particles changed the nucleation form which agreed with the SH model. The electrochemical results were according with SEM results. The EIS （electrochemical impedance spectroscopy） results showed that the addition of dispersant and alumina particles decreased the active area and shifted the initial deposit potential to more negative for their adsorption on the cathode. In other way, the alumina particles might function as nucleation site and refined the crystal to form nano crystal.6. In order to know the effect of magnetic field on the mass transport progress and electron transfer process of Ni-Al₂O₃ deposition, the electrochemical tests were performed in both horizontal magnetic field and vertical high magnetic field. The main factors concluded magnetic flux density and magnetic field direction. Experiment results indicated that the composite deposition process could be affected by the relative orientation of the magnetic field and the electric field. The effect of magnetic field on the electrodeposition was more evident in perpendicular magnetic field than that in parallel magnetic field. In the perpendicular magnetic field, the Lorentz force was in the same direction with natural convection, the Faraday current was enhanced by applied magnetic field. When the Lorentz force was in the opposite direction with natural convection, the Faraday current was inhibited by applied magnetic field. when the magnetic field was applied in parallel with current, the magnetic field also enhanced the Faraday current, but the trend changed at the potential more negative than -0.98V in antiparallel magnetic field. The EIS results showed the applied magnetic field decreased the charge transfer resistance. A different result was shown in vertical high magnetic field. The MHD flow increased the distance of cations from anode to cathode, which increased the electron transfer resistance. At the same time, the MHD flow was blocked by electromagnetic braking force, which could be helpful to decrease the electron transfer resistance.7. The paper showed that the magnetic field enforced the mass transport process and increased the particle content when the Lorentz force was in the same direction with natural convection. Conversely, the opposite result would be got when the Lorentz force was in the contrary direction with natural convection.To conclude, the application of magnetic field in Ni-Al₂O₃ nano composite plating could enhance the mass transport process and electron transfer process and influence the nucleation and growth of composite film. Especially in the parallel magnetic field, the micro-MHD flow field induced by Lorentz force could affect the reduction process distinctly and control the distribution of alumina nano particles in the nickel matrix, which maybe influenced the properties of composite coatings.更多还原