【作者】 卢俊峰；

【导师】 安茂忠；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2007， 博士

【摘要】 电镀银在电子工程领域和装饰领域有着广泛的应用。但其所使用的氰化物镀银液具有剧毒性、严重污染环境和废液处理成本较高等缺点。随着国际环保意识的日益增强,急需开发一种能够替代氰化镀银的无氰电镀银工艺。本论文通过研究几种无氰镀银体系,选定5,5-二甲基乙内酰脲（DMH）为镀液中银离子的配位剂、碳酸钾为导电盐、焦磷酸钾为阳极钝化抑制剂。通过正交实验,结合单因素实验分析,确定较优的镀液组成为:硝酸银30g/L、DMH 100g/L、碳酸钾80g/L、焦磷酸钾40g/L,并确定了适合体系的组合添加剂hit903,其较优用量为10ml/L。考察了施镀过程中镀液组成、温度和pH值等工艺条件对镀层质量、电流效率及极限电流密度的影响。镀液温度愈高、硝酸银含量愈大,极限电流密度愈大。在优化工艺条件下镀液性能与氰化镀液相当,获得的镀层具有较高的硬度、良好的抗变色性能和焊接性能、与基体之间具有很高的结合强度,镀层性能与氰化镀银层相当。研究了脉冲工艺参数对镀层性能和质量的影响,确定了较优的脉冲参数为:占空比40%、脉冲周期3ms、平均电流密度0.6A/dm²。考察了脉冲工艺参数对镀层微观表面形貌和晶体织构的影响。SEM研究表明:脉冲电镀工艺在平均电流密度为0.6A/dm²下能得到光亮、均一的镀层;导通时间和脉冲电流密度的增加会使镀层结晶细致;过长的导通时间和过大的脉冲电流密度使镀层粗糙;关断时间过短,脉动双电层内的金属离子来不及恢复,浓差极化增大,镀层质量变差;关断时间过长,银的晶粒在关断时间内发生重结晶,镀层晶粒变大。XRD分析结果表明:脉冲工艺参数对镀银层的织构有明显影响,在较小的脉冲电流密度和较短的导通时间时,镀层沿（111）和（220）晶面择优取向,在较高的脉冲电流密度和较长的导通时间时,沉积层的择优取向轴为（111）和（311）;关断时间对镀层的织构影响较小。在优化的脉冲工艺条件下施镀获得的镀银层质量好于氰化镀银层质量,优化后脉冲工艺的镀液性能好于氰化镀银镀液性能。通过对DMH体系电极过程的研究,提出了配位离子在电极界面的放电机制,并计算了电极过程动力学参数。研究表明:DMH镀液具有较宽的电化学窗口,在阴极还原过程中,除配位离子的还原外无其它副反应发生;银的电沉积过程经历了三维成核过程,银的阴极沉积过程是受扩散控制的非可逆过程;焦磷酸钾的加入不影响银配位离子在阴极的还原过程,但会降低银阳极的极化;镀液中银配位离子的主要存在形式为AgY2-,在电极界面直接放电的配位离子为AgY。对优化的镀液体系的电极过程动力学参数进行了测定,结果如下:交换电流密度i⁰为1.53×10^-2mA/cm²,阴极过程的表观传递系数α为0.3,电极反应速率常数K为6.72×10^-8cm/s。在此基础上进一步研究了hit903添加剂的作用机理,探讨了hit903添加剂对电结晶过程的影响。结果表明:添加剂的加入能够增大阴极沉积的超电势、抑制阴极峰电流、使成核环负移、减小交换电流密度和电极反应速率常数。计时电流法研究表明:银在玻碳电极上的电结晶过程符合三维连续成核的生长机理;添加剂不改变成核方式,但会抑制晶体的外延生长;添加剂的加入会提高稳态成核速率和饱和晶核密度,但不改变临界晶核原子数;每个独立的原子在界面都可以单独成核。更多还原

【Abstract】 Silver is a metal electroplated for wide applications in both decorative and electronic engineering fields. A cyanide plating bath is most often used, but the cyanide compounds have strong toxicity and a large amount of cost is required for securing safe working conditions and waste treatment. With the enhancement of human environmental consciousness day by day, it is urgent to develop alternative non-cyanide silver solutions for the cyanide silver bath.In the present work, 5,5-dimethyl hydantoin non-cyanide silver electrodeposition baths was choosed as the research baths by comparative study on several non-cyanide silver electrodeposition baths. And potassium carbonate was choosed as conductive salt, potassium pyrophosphate as anodic passivation inhibitor and hit903 as additive. Based on the single factor study, the optimal composition formula was obtained by an orthogonal method. The optimal electrolyte composition is: silver nitrate 30g/L, 5,5-dimethyl hydantoin 100g/L, potassium carbonate 80g/L, potassium pyrophosphate 40g/L, hit903 additive 10ml/L. The influences of operation conditions including electrolyte composition, temperature and pH on current efficiency, limited current density, and coating quality were investigated. Higher the plating temperature and silver nitrate content, higher the limited current density. The coatings obtained in optimum plating conditions possess higher Vickers microhardness, better corrosion resistance and welding properties. The adhesion strength between the coated layer and the substrate is very high. The qualities of coatings and bath obtained with DHM techniques are equal to those obtained with cyanide bath.To improve the qualities of silver coatings, pulse plating process was investigated. Based on the optimal composition and technique conditions by the orthogonal test, the preferred pulse plating parameters were determined and they are: the pulse frequency 40%,the duty cycle 3ms,the pulse average current density 0.6A/dm2. The effects of pulse parameters on the surface morphologies and crystal texture of silver coatings were also studied. SEM analysis indicated that bright and fine silver coatings could be obtained at current density 0.6A /dm2. The increases of pulse on time and pulse current density resulted in more compact deposits, however, the deposits are coarse when pulse on time and pulse current density increased further. Excessive shorter pulse off time leads to the concentration of ions in inner pulse diffusion layer decreasing significantly and concentration difference polarization, as a result, the deposits qualities become poor. Excessive longer pulse off time results in the recrystallization of silver deposits and the crystallite size of the coating becomes larger. XRD analysis indicats that pulse parameters have strong influence on the crystal texture of silver deposits. The silver deposits prepared by shorter pulse on time and low pulse current density exhibit growth orientation of crystal face （111） and （220）, and the silver coatings prepared by longer pulse on time and high pulse current density exhibit growth orientation of crystal face （111） and （311）. And pulse off time has no distinct influence on crystal texture of silver coatings. The quality of silver coating obtained under optimized pulse parameters is better than that of silver coating fabricated with cyanide bath.The mechanism of the reduction of complex ions was put forward and the kinetics parameters of cathodic process were calculated through the investigation of electrodeposition process. The results indicats that the DMH is stable in the range of measurement region and no additional reaction in the reduction process of complex ions. The cathodic deposition process of complex ions is a diffusion-controlled and irreversible process and silver electrodeposition process is also a three-dimensional nucleation process. The addition of potassium pyrophosphate dose not change the cathodic deposition process, but decreases the anodic polarization. The stable species in the bath is AgY2- by calculation from thermodynamical data and the result of electrochemical experiment and a two-step mechanism involving AgY as intermediary was also determined. The kinetics parameters of cathodic process were measured by Tafel and their vales are: i⁰=1.53×10^-2mA/cm²,α=0.3, K=6.72×10^-8cm/s.The influence of the additive on deposition kinetics and mechanism of electrocrystallization process was discussed. The results of Tafel and CV curves showes that the additive decreases the exchange current density values and electrode reaction rate constant, reduces the cathodic current density in the region of the peak and moves the nucleation loop to the negative direction. Chronoamperometric results indicates that the electrodeposition process of silver at GC electrode belongs to a three-dimensional progressive nucleation mechanism and the additive does not change the nucleation mechanism, but inhibits the crystal epitaxial growth. The analysis of nucleation mechanism showes that the additive increases the nucleation rate constant per site and the saturation number density of nuclei, but does not change the number of atoms in the critical nucleus, and the number of atoms in the critical nucleus is 1.更多还原