Vacancies Ordering And Oxidiation Mechanism Of Nonstoichiometric ZrC_x

Owing to the wide homogeneity composition region and high concentration ofvacancies on the octahedral sites of the metalloid sublattice, ZrC_xis often regarded as ahighly nonstoichiometric compound. Under ambient conditions, the ordered carbonvacancies are considered to be the thermodynamic equilibrium, however, using theconventional sintering routes, the synthesis of nonstoichiometric carbides usually leads tothe disordered carbon vacancies. The previous studies indicated that the oxidation of ZrCis diffusion controlled, thus, the presence of carbon vacancies, especially ordered carbonvacancies should have a very significant impact on the oxidation of nonstoichiometricZrC_x. However, the studies focused on the ordering of carbon vacancies, and the effect ofordered vacancies on the oxidation of nonstoichiometric ZrC_xis still lacking up to now.The present study is mainly focused on the realization of vacancy ordering, and the keyrole of ordered carbon vacancies in oxidation process of nonstoichiometric ZrC_xas well asrelative phenomena.Firstly, the nonstoichiometric ZrC_xnanopowders are synthesized by the high-energymilling of pure Zr powder in liquid toluene. By the adjusting of the milling duration andother milling parameter, the C content of nanopowders can be finely controlled. By SPS ofthe mechanochemically synthesized ZrC_xnanopowders, the nonstoichiometric ZrC_0.6hasbeen prepared. The ordered carbon vacancies in the sintered ZrC_0.6are achievedsimultaneously during the sintering process, forming a cubic Zr₂C type ordering phasewith the space group of Fd3m. The cell parameter is determined to be0.9323nm. Themeasurements of TEM and HRTEM indicate that the ordering of carbon vacancies isrestricted to the nanoscale, forming the nanodomains with an average size of³0nm, andseparated by the antiphase domain boundaries. The electron irradiation can induce thedisordering of the ordered vacancies. The first principle calculation indicated that thetwinning interfacial energy in ordered ZrC_0.6is reduced owing to the presence of orderedcarbon vacancies, inducing the presence of {111} twinning. In air and temperature range of25³00℃, thermally activated oxygen diffusion issignificantly facilitated through these vacancy channels. The oxygen atoms diffusedirectly into and occupy the vacancies, producing a cubic Zr₂CO type ordered ZrC_0.6O_0.4with the space group of Fd3m. The cell parameter is determined to be0.93986nm.Similar to the vacancies in ordered ZrC_0.6, the ordering arrangement of O atoms in theordered ZrC_0.6O_0.4is in nanoscale length, thus forming the nano superstructural domainswith irregular shapes. The large amount of {111} twinning are also observed in orderedZrC_0.6O_0.4. Slightly diffusion of superfluous oxygen atoms can induce the long rangeordering of oxygen atoms transform to short range ordering, however, at the elevatedtemperature higher than300℃, the superfluous diffusion of oxygen atoms can induce theamorphization of ordered ZrC_0.6O_0.4, forming amorphous ZrC_0.6Oy>0.4layer on the surfaceof powders. The amorphous formation is recognized to originate from diffusion ofsuperfluous oxygen atoms into Zr-tetrahedral centers in the surface area, thus leading tosevere distortion of lattice. Under electron beam irradiation, the amorphous ZrC_0.6Oy>0.4layer transforms to a cubic ZrO_2-xlayer with the same orientation as the underlyingordered ZrC_0.6O_0.4. Moreover, the orthorhombic-like ZrO_2-xembedded at locally areas isrecognized inside the cubic ZrO_2-xlayer.In air and the temperature range of380⁶20oC, the vacancies ordered ZrC_0.6can befast oxidized to predominant spherical tetragonal ZrO₂nanocrystals. By annealing of theas-prepared tetragonal ZrO₂nanocrystals at higher temperature, the growth of thosenanocrystals owing to the coalescence of adjacent nanocryatals has been observed. Threetypes of {011}-specific twins, i.e., single, lamellar, and fivefold cyclic twins, arerecognized in those coalescence induced nanoparticles. The formation of single andlamellar twins is occasional and occurs via the coalescence of adjacent nanocrystals onwell-developed {011} facets of tetragonal ZrO₂nanocrystals. The formation of dominantcyclic twins originates from the coalescence on the mismatched surfaces, and the emissionof partial dislocations and plastic deformation are identified to play the key role. In someZrO₂nanoparticles，the presence of orthorhombic phase with space group of Pbca andPbcm are identified with the helps of HRTEM and XRD. The twinning boundary innanoparticles before the martensitic phase transformation and the microstrains induced by the phase transformation play the key role in the formaiton and stabilization of observedorthorhombic ZrO₂.The present study confirm the presence of cubic Zr₂C type ordered phase, as well asthe influence of ordered carbon vacancies for twinning interfacial energy innonstoichiometric ZrC_x. The investigations focused on the oxidation mechanism andproducts reveal the key role of ordered carbon vacancies in the oxidation process ofnonstoichiometric ZrC_x, confirm the presence of intermediate phase of ZrC_xOy. Based onthe quickly oxidation of ordered ZrC_0.6at low temperature, a routine for preparation oftetragonal ZrO₂nanopowders. The twinning structures in annealing induced tetragonalnanoparticles are also investigated, and a formation mechanism is proposed. By themearsurments of XRD and HRTEM, the crystal structures and the formation mechanismof observed orthorhombic ZrO₂are also investigated.