Melting point in systems ZrC-HfC, TaC-ZrC, TaC-HfC

Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system

“…Agte and Altherthum4 reported a maximum melting temperature ( T m ) for Ta 0.8 Hf 0.2 C of 4213 K, and this was later confirmed by Andriievski et al 6. who measured it as 4263 K. The temperature trend with a maximum melting point at an intermediate composition was not confirmed in the work by Rudy7.…”

“…More recently, Hong and van de Walle9 predicted the melting temperatures using density functional theory (DFT) and their calculations suggest that the melting temperature of HfC 0.81 is 3962 K, higher than TaC 0.88 at 3830 K. Also these calculations were consistent with a local maximum within the solid solutions, for a composition close to Ta 0.8 Hf 0.2 C at 3920 K. The work of Hong and de Walle was particularly useful, in that it identified precise physical mechanisms behind the effects of carbon hypostoichiometry and alloying on the melting temperature of a given composition. In addition, HfC 0.56 N 0.38 was reported as the compound with the highest melting temperature at 4141 K. In summary, the uncertain and contradictory results on the melting points reported suggest that: (i) thanks to the entropic contribution of lattice defects to reducing the free energy of the solid phase, slightly hypostoichiometric monocarbides might have melting temperatures higher than their solid solutions789; (ii) because of the Fermi energy position in the mixed carbides, a maximum melting temperature within the solid solutions (TaC-HfC) might exist469. This calls for further experimental work perhaps with other techniques than were previously available.…”

Spark plasma sintering of TaC–HfC UHTC via disilicides sintering aids

“…Also it has the highest melting point of the known refractory substances (4000 • C). [6][7][8] Sintering of bulk TaC or HfC is difficult due to their high melting point. Employing smaller particle size for TaC, high temperature and sintering aid (C, B 4 C, MoSi 2 , TaSi 2 ) are necessary to achieve densities more than 95% by pressureless or hot-press sintering routes.…”

Prediction of the material with highest known melting point fromab initiomolecular dynamics calculations

“…Starting from stoichiometric HfC, the melting temperature increases along with carbon deficiency, until it reaches a maximum at the congruent melting point, near 45 atomic % C (HfC 0.82 ). This feature explains why HfC undergoes carbon loss when it is heated and melted [29]: it remains in the solid state and loses carbon as the temperature increases. Further decrease in carbon composition leads to a drop of melting temperature.…”

“…While Because of the rapid evaporation of carbon at high temperatures, it is difficult to assign the measured melting point to the correct composition in experiments [12,29]. The maximal melting temperature in the Hf-C system falls in the HfC 0.85−0.95 region [7,9,10,12,29], while Ta-C has a strong tendency to lose carbon and the melting point maximum is near TaC 0.8−0.9 [12,14,29]. The Hf-Ta-C system contains HfTa 4 C 5 , which has long been considered the most refractory substance known to date [17].…”