The starting nanopowders of non-stoichiometric zirconium carbide (ZrCx) were fabricated via milling Zr powders in toluene for different dwell times. The carbon content was determined to depend on the milling time and the used amount of toluene. The bulk non-stoichiometric ZrCx with different x were prepared by spark plasma sintering of the obtained ZrCx nanopowders. The microstructural features of a sintered ZrC0.6 sample were investigated via the measurements of XRD, TEM, and HRTEM. It was found that the carbon vacancies have an ordering arrangement in C sublattice, forming a Zr2C-type cubic superstructural phase with space group of Moreover, it was observed that the superstructural phase exists in nano-domains with an average size of ~30 nm owing to the ordering length in nanoscale. During the heating treatment in air, it was recognized that the diffusion of oxygen atoms is significantly facilitated through the ordered carbon vacancies. For the heating treatment at low temperature (<300°C), the oxygen atoms diffuse easily into and occupy the ordered carbon vacancies, forming the oxy-carbide of ZrC0.6O0.4 with ordered oxygen atoms. At the heating temperature higher than 350 °C an amorphous layer of ZrC0.6Oy>0.4 was identified to be formed due to the diffusion of superfluous oxygen atoms into Zr-tetrahedral centers. Inside the amorphous layer, the metastable tetragonal zirconia nanocrystals are recognized to be gradually developed.
TopIntroduction
As one of the important refractory materials, zirconium carbide (ZrC) exhibits many excellent properties such as high hardness (Pierson, 1996), good corrosion resistance (Pierson, 1996; Oyama, 1996), good electrical and thermal conductivity (Toth, 1971), the low neutron absorption or scattering cross sections and low damage under irradiation (Ogawa & Ikawa, 1982), etc. Owing to these attractive properties, it demonstrates great potential in the technological applications, such as cutting tools, high-speed aircraft leading edges (Van Wie et al., 2004) operating in high or ultra-high temperature, the thermal-field emitters (Yada, et al., 1989), etc. In past decades, intensive investigations have been carried out on the properties and applications of ZrC. The nonstoichiometric nature of most obtained bulk zirconium carbide has been cognizant (Pierson, 1996), and some ordered phases have also been predicted theoretically (Gusev & Rempel, 1993). However, the experimental studies on the ordered carbon vacancies are pretty rare to date. Furthermore, the studies on the oxidation of nonstoichiometric ZrCx with x<1 are very insufficient, especially for carbon vacancies ordered phase.
In this chapter, we will introduce the preparation and structural features of carbon vacancy ordered ZrC0.6. The oxidation process and products for this ordered phase are also discussed with the help of XRD, TEM, and HRTEM measurements.