Carbides and Nitrides of Zirconium and Hafnium

Abstract

Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40–60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions.

Early transition metal nano-carbides and nano-hydrides from solid-state metathesis initiated at room temperature

“…Early transition metal carbides are known to be nonstoichiometric due to carbon vacancies: zirconium and hafnium carbides are solid solutions with a continuum of stoichiometries from MC 0.5 to MC 1.0 with small variations of the cell parameters (respectively 0.17 % and 0.5 % of variation between the two extremes). [5] Here, the peaks are broadened due to the small size of the crystallites and the precise stoichiometry of the carbide cannot be stated. [26] All the powders were washed with ethanol and water and then exposed to air, surface oxidation is therefore expected.…”

“…[3,4] Most of the TMC syntheses imply an extended time of reaction (up to one week) at elevated temperatures (> 800 °C) between the metal precursor and a carbon source such as graphite, a polymer or light hydrocarbons. [5] To limit the oxidation of the material, the reaction is usually conducted under a reductive or an inert atmosphere. The high temperatures and the long reaction times are critical for these syntheses to overcome the solid-state diffusion barrier of carbon into metal.…”

Erosion of refractory carbides in high‐temperature hydrogen from ab initio computations

“…In this regard, a limitation for refractory carbides and their hydrogen decomposition products is the availability of basic thermodynamic data. A number of heritage studies on carbides provide values of thermal expansion, heat of formation, and partial thermodynamic state function information ( e.g ., enthalpy and free energy) up to 2600–3000 K 15–52 . Because direct experimental quantification of refractory carbide properties above this temperature range is limited, extrapolations of fits are employed to predict high‐temperature behavior.…”

Microstructure Evolution in ZrCx with Different Stoichiometries Irradiated by Four MeV Au Ions

“…Zirconium carbide (ZrC) has been considered as a Tri-structural iso-tropic (TRISO)-coating fuel particle, fuel cladding or an inert matrix material, due to its high melting point, high thermal conductivity, low neutron absorption cross-section and excellent resistance to attack by fission products [6,7,8]. As a member of the family of transition metal carbides, ZrC has an NaCl crystal structure which is stable over a relatively wide compositional range of 0.6 to 1.0 [9].…”