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

Abstract

TaC, HfC and their solid solutions are promising candidate materials for thermal protection structures in hypersonic vehicles because of their very high melting temperatures (>4000 K) among other properties. The melting temperatures of slightly hypostoichiometric TaC, HfC and three solid solution compositions (Ta1−xHfxC, with x = 0.8, 0.5 and 0.2) have long been identified as the highest known. In the current research, they were reassessed, for the first time in the last fifty years, using a laser heating technique. They were found to melt in the range of 4041–4232 K, with HfC having the highest and TaC the lowest. Spectral radiance of the hot samples was measured in situ, showing that the optical emissivity of these compounds plays a fundamental role in their heat balance. Independently, the results show that the melting point for HfC0.98, (4232 ± 84) K, is the highest recorded for any compound studied until now.

The role of ceramic and glass science research in meeting societal challenges: Report from an NSF‐sponsored workshop

“…Theory and computational modeling are essential tools to guide the discovery process. The value of theory is illustrated by recent work that provided a fundamental explanation for the reported maximum in melting point for rock‐salt solutions of TaC–HfC at ~(Ta 0.8 Hf 0.2 )C 0.875 . The authors further predicted that compounds in the Hf–C–N system could exhibit melting points at least 200°C higher than in Ta–Hf–C, although this prediction is yet to be validated experimentally.…”

“…While characterization of materials under combined extremes is still challenging, progress has been made in measuring behaviors in simplified environments typically dominated by a single extreme condition. For example, one can measure melting temperatures well over 3000°C using new laser techniques, as demonstrated recently on HfC 0.98 , (3959°C±84°C) . It is also possible to measure strength, thermal diffusivity, heat capacity, coefficient of thermal expansion, and electrical conductivity at very high temperatures, as well as material responses in extreme heat fluxes .…”

“…The value of theory is illustrated by recent work that provided a fundamental explanation for the reported maximum in melting point for rock-salt solutions of TaC-HfC at~(Ta 0.8 Hf 0.2 )C 0.875 . [194][195][196][197] The authors further predicted that compounds in the Hf-C-N system could exhibit melting points at least 200°C higher than in Ta-Hf-C, although this prediction is yet to be validated experimentally. Methods based on DFT are now able to calculate mechanical, electronic and thermophysical properties of ceramics at low temperatures.…”

“…For example, one can measure melting temperatures well over 3000°C using new laser techniques, 210 as demonstrated recently on HfC 0.98 , (3959°CAE84°C). 195 It is also possible to measure strength, 211 thermal diffusivity, 212 heat capacity, 203 coefficient of thermal expansion, 204,213 and electrical conductivity 214 at very high temperatures, as well as material responses in extreme heat fluxes. 215,216 A limiting factor often becomes the stability and inertness of the sensors needed to measure the quantities of interest, both in the environment as well as in contact with the specimen.…”

Bond characteristics, mechanical properties, and high‐temperature thermal conductivity of (Hf1−xTax)C composites

“…There have been some theoretical studies of the elastic, mechanical, and electronic properties of the composites; however, only limited information is available. Besides, although (Hf 1−x Ta x )C solid solution composites have been synthesized, there are few data on their elastic, mechanical, electronic, and melting behaviors due to experimental difficulties arising from the high‐temperature stability of the individual carbides, TaC and HfC . Furthermore, there are no studies of the minimum high‐temperature thermal conductivity of these carbides for high‐temperature applications, to our best knowledge.…”