【作者】 王娟；

【导师】 凤仪；

【作者基本信息】 合肥工业大学 ， 材料学， 2009， 博士

【摘要】 碳纳米管具有优异的力学性能和良好的导电导热性,是理想的复合材料增强体。本文通过对碳纳米管表面镀覆一层银,增加碳纳米管与基体的界面结合强度、提高碳纳米管的分散性,从而改善复合材料性能。由于碳纳米管尺寸小、曲率大、易缠结,对其表面进行化学镀银比较困难。研究发现,经过研磨、氧化、敏化、活化后,可以提高碳纳米管的表面活性,最终在碳纳米管表面成功镀覆一层比较均匀的银层。采用粉末冶金方法制备碳纳米管-银-石墨复合材料,初压压力200MPa,在H2保护气氛下烧结并保温1h,复压压力400MPa制得复合材料。复合材料中添加石墨和碳纳米管保证了复合材料具有自润滑作用以及良好的强度、硬度。研究了石墨和碳纳米管含量变化对复合材料组织性能的影响。首先,保持碳纳米管质量百分比1%不变,制备石墨质量百分比分别为8%,10%,13%,15%和18%的CNTs-Ag-G复合材料。研究发现,随着复合材料中石墨含量的增多,材料的相对密度、布氏硬度值降低,电阻率升高。并且复压后的材料性能明显高于复压前。其次,保持银体积百分比65%不变,使用碳纳米管部分取代石墨,其中碳纳米管体积百分比分别为0%,2%,4%,6%和8%。研究发现,当复合材料中碳纳米管的含量较少时,材料的相对密度和布氏硬度随碳纳米管含量的增加而增大,而电阻率则随碳纳米管含量的增加而持续下降。当含量超过6%后,复合材料的相对密度和硬度有下降趋势,这主要由于碳纳米管在复合材料中分布不均匀所致。研究了电流极性、电流密度、石墨含量和碳纳米管含量对复合材料接触电压降的影响。研究发现,在电磨损过程中,正负电刷接触电压降变化趋势相同—先上升然后趋于稳定,且负刷的接触电压降大于正刷的接触电压降。随着电流密度的增大,接触电压降略有上升,但与电流密度之间为非线性关系,符合电机用电刷使用要求。电刷的接触电压降随复合材料中石墨质量百分含量的增大先上升最后趋于稳定,随复合材料中CNTs的体积百分含量的增大先上升后下降。研究了石墨含量、碳纳米管含量和电流密度对复合材料摩擦系数的影响。随着复合材料中石墨质量百分含量的增多,电刷的摩擦系数逐渐降低,并且电摩擦系数远大于机械摩擦系数。这是因为随着石墨含量的增多,复合材料在磨损过程中形成的润滑膜更加完整,摩擦发生在润滑膜层,因此摩擦系数减小;由于电磨损中润滑膜容易被破坏,因此电磨损中摩擦系数较大。复合材料的摩擦系数随材料中CNTs体积百分含量的增加先上升后下降,碳纳米管含量6%时,摩擦系数最大。电磨损过程中的摩擦系数明显高于机械磨损的摩擦系数,且两者之间有着倍数的差别。随单位面积上电流密度的增加(0~25A/cm2),摩擦系数上升;且摩擦初始阶段的摩擦系数较大,随时间推移,摩擦系数逐渐减小,最后稳定在一定范围内。研究了石墨含量、碳纳米管含量、电流极性和电流密度对复合材料磨损量的影响。随石墨含量的增加,电磨损量先下降后上升,石墨质量百分含量在13%左右磨损量最低。这是由于随着石墨含量的增加,复合材料的磨损机制发生转变造成的。碳纳米管体积含量对复合材料磨损量的影响表明,当CNTs体积百分含量小于6%时,材料的耐磨性提高,复合材料磨损量随CNTs体积百分含量的增加而下降,此时碳纳米管可在复合材料中均匀分布;当CNTs含量超过6%后,磨损量有增加的趋势,这是因为碳纳米管含量过多容易在复合材料中缠结在一起,不能起到有效的增强作用。随着电流密度的增大,电刷磨损量增大。电磨损过程中复合材料的磨损存在极性差异:正刷磨损量始终大于负刷磨损量,且随着电流密度的增大,正刷磨损量成倍增长,负刷磨损量则与电流密度关系不大,电流密度越大,极性差异越明显。更多还原

【Abstract】 CNTs are thought as perfect enhancements for composites for their attractive mechanical property and good electrical and thermal conductivity. In this paper, the CNT was covered with a layer of silver to enhance the strength of interface between CNT and Ag matrix and improve the distribution of CNT in composites. Because of small size and big curvature, CNT was apt to twist togther and very difficult to coat a layer of silver on the surface of CNT. It was found that, the activation of CNT would be enhanced after grinding, oxidization, sensitization and activation.The CNTs-Ag-G composites were fabricated by means of powder metallurgy method. The process included mixing powder, pressing at 200MPa, sintering for 1h in pure H2 protective atmosphere at 700℃and repressing at 400MPa. Adding graphite and CNTs could guarantee the self-lubricating property and good strength and hardness of composites.The effects of graphite and CNTs content on the microstructure and performances of composites were studied. Firstly, keeping CNTs mass fraction as 1%, the composites were added graphite with mass fraction of 8%, 10%, 13%, 15% and 18%, respectively. The results showed that the relative density and Brinell hardness decreased while the electrical resistivity increased with the increase of graphite content. And the performances of composites after repressing were much better than composites before repressing. Secondly,remain silver volume fraction as 65%, using carbon nanotubes replaced graphite partially and the CNTs volume fraction were 0%, 2%, 4%, 6% and 8%, respectively. The resultes showed that when having a little CNTs, the relative density and hardness was increased with the CNTs volume fraction while the electrical resistivity continuously decreased. While exceeding 6%, aggregation of CNTs brought about the decrease of relative density and Brinell hardness.The influences of elecrrical polarity, current density, content of graphite and CNTs on contact voltage drop were discussed. The results showed that, during electrical wear process, the contact voltage drop of both positive and negative brushes had the same trend: be stable after first increase, and the contact voltage of negative brush was always bigger than positive brush. Contact voltage drop had a slight increase (non-linear relationship) with the increase of current density which meet the requirements of electric power brush. The contact voltage drop first increased then tended to be stable with the increasing of graphite mass fraction. Thirdly, the influence of CNTs content on contact voltage drop of composite was studied. The results showed that, the contact voltage drop increased with the increasing of CNTs content, and it would decreased with continuous increasing of CNTs content.The effect of graphite and CNTs content and current density on the friction coefficient of composites was studied. The results showed that, with the increasing of graphite content, the friction coefficient persisted decrease and the friction coefficient in electrical wear was bigger than that in mechanical wear. This because that, with the increasing of graphite mass fraction, the lubricating film beame more integrity, and this layer of film was easier to be ruptured in electrical wear. With the increasing of CNT content, the friction coefficient of composites was decrease after first increase and the maximum value was obtained when adding 6vol% CNTs. The electrical friction coefficient was mutiple bigger than that of mechanical friction coefficient. With the increase of current density the friction coefficient had a slight incrase and got a bigger value in the initial stage of friction process and then decrease until stabile.The wear mass loss of composites influenced by content of graphite and CNTs, electrical polarity and current density was also studied. The results showed that, wear mass loss of compostie was increase after first decrease with the increasing of graphite mass fraction, and the minimum value was obtained when composite having about 13wt% graphite. This was caused by the transition of wear mechanism with increasing of graphite content. The wear resistance was improved when adding a little CNTs (less than 6vol%) and the wear mass loss decreased rapidly. While the content of CNTs bigger than 6vol%, wear mass loss had a tendency to increase because of the appearance of tangled CNTs in matrix and weakness of CNTs reinforcement. Furthermore,the wear mass loss increased with the increasing of current density. Polar differences were existence in electrical wear, that is, the wear mass loss of positive brush was always greater than the negative brush and with increasing of current density the mass loss of positive brush increased multiply while the mass loss of negative brush had a little relationship with current density. And this kind of polar difference became more apparent with the current density increases.更多还原