Carbon vacancy ordering in zirconium carbide powder

Abstract

Ordered carbon vacancies were detected in zirconium carbide (ZrCx) powders that were synthesized by direct reaction. Zirconium hydride (ZrH2) and carbon black were used as starting powders with the molar ratio of ZrH2:C = 1:0.6. Powders were reacted at 1300°C or 2000°C. The major phase detected by x‐ray diffraction (XRD) was ZrCx. No excess carbon was observed by transmission electron microscopy (TEM) in powders synthesized at either temperature. Ordering of the carbon vacancies was identified by neutron powder diffraction (NPD) and further supported by selected area electron diffraction (SAED). The vacancies in carbon‐deficient ZrCx exhibited diamond cubic symmetry with a supercell that consisted of eight (2 × 2 × 2) ZrCx unit cells with the rock‐salt structure. Rietveld refinement of the neutron diffraction patterns revealed that the synthesis temperature did not have a significant effect on the degree of vacancy ordering in ZrCx powders. Direct synthesis of ZrC0.6 resulted in the partial ordering of carbon vacancies without the need for extended isothermal annealing as reported in previous experimental studies.

The effect of oxygen impurities on the stability and structural properties of vacancy-ordered and -disordered ZrCx

“…Additionally, Zr 2 C can tolerate more oxygen than other ordered phases due to its larger ordering energy at a given oxygen concentration. This is consistent with the Zr 2 C phase being the most-oen experimentally synthesised ordered zirconium carbide, being reported twice as oen 4,6,8,9,[11][12][13] as all other ordered structures combined. 4,7,8,10…”

“… 3 Various vacancy-ordered phases have been intermittently reported in the experimental literature but there is no consensus regarding phase stability. Goretzki observed a Zr 2 C superlattice, 6 Obata and Nakazawa unsuccessfully attempted to fabricate Zr 2 C, but observed Zr 4 C 3 , 7 de Novion and Maurice reported the Zr 2 C phase, 8 Karimov et al reported Zr 2 C, 9 Khaenko et al reported possible trigonal Zr 8 C 5 , 10 Hu et al observed Zr 2 C superstructures, 11 Wei et al reported Zr 2 C, 12 Zhou et al fabricated Zr 2 C, 13 and Rana et al reported possible systematic vacancy ordering. 4 Theoretical calculations confirm that long-range superstructural ordering of vacancies is expected at low temperature.…”

Vacancy ordering in substoichiometric zirconium carbide: A review

“…The orderdisorder phase boundaries calculated by Gusev and Rempel separate the regions where some degree of long-range ordering occurs and where a fully disordered solid solution is expected. The second-order reaction, Zr 2 C→ ZrC x , has a maximum transformation temperature of 1219 K (946 phase is consistent in structure with the cubic and trigonal experimental observations, 21,[25][26][27]31,34 and the orderdisorder transition temperatures are consistent with the transition at ∼1170 K (∼897 • C) for ZrC 0.63 measured by Karimov et al 26 The disordering temperature is also consistent with measurements by Obata and Nakazawa of 1340 K (1067 • C) for ZrC 0.70 , 22 although the Zr 4 C 3 structure determined in that work is not found in the calculated phase diagram. Despite Zr 3 C 2 and Zr 6 C 5 not having been observed experimentally, Gusev et al believed them to be stable on the basis of comparison with the observed analogous Ti 3 C 2 phase and investigations of this composition range.…”

Plasma-assisted preparation of nano-(ZrC, ZrO2)@carbon composites from Zr-loaded sulfonated styrene–divinylbenzene copolymers

“…In principle, if the carbon loss during carbonization (degradation into volatile organic materials or CO/CO 2 ) is less than 58 mass % for 2-DVB-Zr samples or 66% for 8-DVB-Zr samples, then the residual carbon content in the solid phase is larger than is required to form stoichiometric ZrC. Depending on the amount of volatile carbon compounds (it depends on pressure, temperature, time and heating method) and the carbon loss during pyrolysis, the residual Zr/C ratio in nano-ZrO 2 @C and nano-ZrC@C composites can be increased and only ZrC products with a carbon vacancy (ZrC 0.63−0.98 [23]) may be created.…”

“…Zirconium carbide ceramic materials formed in direct reactions of Zr or Zr compounds and carbon or carbon precursors, even in the case of the most reactive ZrH 2 , always contains some carbon vacancy [16]. The reaction of carbondeficient ZrC x structures with the free carbon results in the formation of non-stoichiometric Zr 1-x C (carbon-rich) materials and increases their sinterability [23].…”