Research On Microstructure And Properities Of Tungsten-Copper Composite Materials Doped With Graphene

Tungsten-copper (W-Cu) alloys are widely used in electrical and electronic fields as electric contact materials for high voltage electric switches due to its high hardness, high arc erosion resistance and fusion welding performance and outstanding thermal and electrical properties. In recent years, long distance and extra high voltage transmission are put forward with the distribution of energy. In particular, high-voltage power transmission capacity of ultra-high voltage development and miniaturization tendency of new electronic products put forward the new performance requirements of contact material, such as both high electrical performance and excellent anti-ablation property. In order to improve the comprehensive properties of new contact materials, graphene was successfully prepared by chemical reduction of GO using thiourea dioxide (TD) as a green reducing agent firstly, and then an attempt was made to fabricate graphene/W70Cu30 composites by ball milling and pressureless infiltration sintering technology. The graphene oxide after reduction with thiourea dioxide has been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman photoelectron spectroscopy and transmission electron microscopy (TEM). The factors affecting reduction degree of graphene were discussed in the process of reduction, such as reduction time,temperature and the amount of reducing agent. Also, microstructure and properties on graphene/W70Cu30 composites were characterized by transmission electron microscopy(TEM), X-ray diffraction (XRD), Raman spectroscopy, Optical microscopy (OM) and HB-3000 hardness tester, respectively. Finally, the reduction mechanism of graphene used TD reductant and graphene/W70Cu30 contacts arc breakdown and dominant mechanism were discussed,respectively. Experimental results show:1. When the reduction temperature was 85 ℃, the reduction time was 1.5 h, reducing agent amount was 0.2 ml, little defects in the graphene morphology as transparent as chiffon were observed. The green reducing agent in the work has lots of advantages over conventional reductant, such as low cost, high efficiency and no pollution on environment. It was founded that the stronger reduction capacity of TD was attributed to the synergistic effect of its hydrolysis products, such as HSO3- and catalyze the ring-opening reaction in the alkaline solution.2. The graphene could remain after ball milling and sintering process for composite powders and bulk materials, respectively. But, the carbides (like WC and W2C) were formed in bulk composites when the graphene content was 1.0 wt%. It revealed that some of graphene would react with W during the sintering process, but the graphene coated with Cu phase in W70Cu30 alloys still keep intrinsic structures. And W grain size was efficiently refined with addition of graphene. The maximum grain size of W particles can be refined from intial ～45.50μm to ～14.67μm. Also, graphene can activate sintering WCu alloys.3. The relative density of graphene/W70Cu30 composites enhanced with the content of graphene increasing. When the graphene content was 1.0 wt%, the relative density of composites reached up to 98.4%. The incorporation of graphene into W70Cu30 alloy gradually increases the hardness of composites,with a value of HB208, which was enhanced about 21%compared with the pure W70Cu30 alloys when the graphene content was 1.0 wt%. The electrical conductivity of composites increased gradually firstly and decreased sharply with the increase of graphene content. When the 0.5 wt% graphene was added to the W70Cu30 alloys,the maximum conductivity reached～46% IACS.4. The mass loss of WCu contact material tended to decrease firstly and then increase with a graphene content increasing during the arc ablation breakdown process. When the graphene content was 0.5 wt%, the smallest weight loss of WCu contact was founded, with a value of 0.42 mg, which suugested the properties of arc erosion was enhanced around 30%compared with W70Cu30 alloys without any additives. Also, breakdown strength of W70Cu30 alloy contacts increased firstly and then decreases with the increase of the graphene mass fraction. Compared with pure W70Cu30 alloys (5.5×106 V/m), a maximum breakdown strength value was 7.0×106 V/m at a graphene mass fraction of 0.5 wt%,which reached up to～27.3%.5. When the arc breakdown took place, the arc ablation was firstly transferred from the Cu phase to the graphene surface with a lower work function. At the same time, the surface morphology of W70Cu30 contacts with graphene addition have a slight splash of molten copper and a small, flat cathode craters, thus, avoiding the concentration erosion on the surface of contacts. The main ablation mechanisms of W70Cu30 contact with graphene addition was considered to be electron transition, energy consumption and difference work function of W,Cu, graphene, respectively.