Fabrication And Tribological Properties Of WSe₂ And WSe₂/C Nanostructures

Hexagonal sandwich-like structure(so-called sandwich structure) tungsten diselenide(WSe2) materials have wide applications in solar cell materials, high temperature solid lubricants and low thermal conductivity materials (i.e., thermal insulating materials), due to its high efficient optical transmission semiconductivity, high temperature resistance, low thermal conductivity and special lubrication. However, preparation cost of WSe2 is very high. Moreover,WSe2 is oxidized easily at 350℃, when used as solid lubricants with high resistance temperature under dry friction in oxygen atmosphere. Obviously, it should be extremely important that investigation into the material itself in theoretical research and practical application, involved with its nanoscale and compounding made to achieve the optimization or give new optical, tribological and thermal properties.Moreover, carbon material has widely promising application in stabilization (especially antioxidation) and strengthening of the others, due to its light, low price, inert (such as acid and alkali resistance, air stability and certain thermal stability), biocompatibility and good conductivity.In this thesis, based on the above cases,WSe2 nanostructures were synthesized by two environment friendly technologies, including the solid reaction from the elements W and Se directly, and the hydrothermal reaction using sodium tungstate(Na2WO4·2H2O), Se, reductant and others successfully. Furthermore, WSe2/C composite was obtained by the introduction of the carbon source, such as graphite, small organic molecule compound sucrose and organic polymer maleic anhydride graft ethylene/octene copolymer (POE-g-MA), into the starting materials. Structure, morphology, reaction kinetics characteristics, hot oxygen stability and tribological properties of as-fabricated samples were studied in detail using the X-ray diffractometer, SEM, TEM, Raman spectrometer, differential scanning calorimeter (DSC), thermogravimetric (TG) analyzer and multi-purpose friction and wear tester.First of all, solid phase synthesis of sealed tungsten-selenium system was investigated at various temperatures such as 600,700 and 800℃, respectively. The results indicated that all as-obtained samples are sheet-like and nanoscale, and get irregular due to the rapid growth of WSe2 grains at elevated temperature. However, as-prepared WSe2 nanostructures via hydrothermal method at 200℃were flower-like and formed by the assembling of quantities of WSe2 nanosheets with 10 nm in diameter. The tribological properties of WSe2 synthesized by the two methods added into paraffin in steel/steel interaction were also investigated by a UMT-2 ball-on-disc tribometer. The results suggested that the addition of the nanostructures can greatly improve the reducing-friction and anti-wear properties of paraffin, showing good lubricating performance, but the flower-like nanostructures present much better.Secondly, it was found that size and morphology of the WSe2/C composites were greatly affected by the carbon source content in the starting materials in a sealed reactor, besides reaction temperatures. The foundings are as follows.(1) At conditions of 800℃and sealing, pure sugar and POE-g-MA can be pyrolysized into the layered squalid particles and uneven carbon micro-balls;(2) Among WSe2/C composites prepared at 800℃using various content of sucrose and POE-g-MA, respectively, carbon-encapsulated WSe2(WSe2@C) nanostructures were obtained at proper carbon source content. In contrast, POE-g-MA as carbon source presented very good encapsulation ability than sucrose. However, when graphite was used as carbon source, only stacking WSe2 nanosheets were formed and reduced in size with the increase of graphite content;(3) In-situ carbon-encapsulation mechanism was investigated by taking the temperature and carbon source species into consideration, combined with thermal dynamic analysis of those raw mixtures in the sealed system. The results show that synthesis temperature of core materials and thermal stability of carbon source play key roles in the formation of the nanocomposites with core-shell structure. Specifically, core materials must be formed before effective carbon source was decomposed and carbonized;(4) Moreover, thermal stability of the resultant WSe2@C nanostructures, obtained using sucrose and POE-g-MA as carbon source, respectively, was valued by TG-DSC in the air atmosphere. The results both show good improvements with corresponding increase in oxidation peak value by 30℃and 50℃, compared with pure WSe2;(5) As base oil lubrication additives, WSe2/C is very effective in reducing frictional coefficient and enhancing anti-wear, in contrast to pure WSe2, showing good lubrication performance; and (6) Tribological properties of the resultant nanostructures filled PEEK, with good wear resistance and high frictional coefficient, were studied and compared with those of commercial pure PEEK under dry and water lubrication conditions, respectively. The results indicate that WSe2/graphite and WSe2@C (POE-g-MA as carbon source) are good anti-wear and friction-reducing fillers of PEEK under dry condition. Moreover, WSe2/graphite nanostructures present even better under water lubrication.In general, those researches with the aim of development of solid-state and hydrothermal processes not only provide cost-effective and environmently friendly technologies for such transition-metal chalcogenides as MoSe2 and WS2, as well as WSe2, but also make beneficial exploration for the formation mechanism of carbon-encapsulated materials, morphology and size control, improvement in thermal-oxidative stability and tribological properties. All those provide for theory basis and establish experimental basis for more deep research and better and more extensive application carbon-encapsulated material.