The composition versus temperature phase diagram constitutes the most basic information for each car­bide system, fundamental to correlate thermophysi­cal, thermodynamic, and chemical data ofcompounds in a consistent way. Thus, phase stability data are first given for each actinide carbide system, followed by a review ofthe available information on physicochemi­cal data.

Although the general properties have been assessed, especially for the most studied systems, Th-C, U-C, and Pu-C, doubts still remain about the effective stability or ‘meta’-stability of certain crucial phases (e. g., UC2 at room temperature). The current phase diagrams, often completed with newer data and assessed by more recently developed ther­modynamic optimization methods (CALPHAD), seem to generally, but not always, confirm the data obtained in the 1950s-1960s with traditional thermal analysis techniques. The discrepancies are sometimes linked to the deviation of the samples investigated from an ideal behavior, mostly due to oxygen and nitrogen contamination, a well-known and common issue related to carbides.

A short discussion of the most common actinide carbide oxides and carbide nitrides is, therefore, pre­sented, with the goal of providing a hint of the main effects ofoxygen and nitrogen additions on the phys­icochemical properties of pure carbides.

2.04.1.2.2 Preparation

Actinide mono — and dicarbides for research purposes are preferentially prepared by arc-melting a mixture of metal and graphite in the right proportions. This process is normally performed under ^1bar of helium or argon. Special care is needed to avoid oxygen, nitrogen, and water impurities in the furnace.

	2

Th

Figure 2 DFT calculations of (a) 1 — charge density map and 2 — charge density profiles along the Th-C and Th-Th bonding lines in the (100) plane of face-centered cubic ThC (reproduced from Shein, I. R.; Shein, K. I.; Ivanovskii, A. L. J. Nucl. Mater. 2006, 353, 19-26). (b) Charge density map in the (110) plane of tetragonal p-ThC2 (reproduced from Shein, I. R.; Ivanovskii, A. L. J. Nucl. Mater. 2009, 393, 192-196).

The preparation of oxygen and nitrogen-free car­bides is hardly possible.

Probably the most used method for industrial applications is the carbothermic reaction of AnO2, based on a reaction of the type:

UO2 + 3C! UC + 2CO [I]

normally performed under vacuum (1.25 x 10~5bar) at 1700-1850 K for 4 h.

Other possible preparation methods are reaction of An hydrides with carbon, aluminothermic reaction of AnF4, pyrolytic reaction of AnCl4 with CH4, and An-Hg amalgam distillation in a hydrocarbon atmosphere. Single crystals have been obtained by electron-beam melting, quenching, and anneal­ing of polycrystalline samples. Potter25 showed that carbothermic reduction of PuO2 cannot yield oxygen — free Pu monocarbide, because the very high Pu pressures corresponding to the Pu2C3-PuC!_xOx equilibrium would lead to the formation of Pu2O3 or Pu2C3 in equilibrium with PuC1_x.

The preparation ofsesquicarbides is more compli­cated. Th2C3 and U2C3 have been obtained with complex experimental procedures, whereas the prep­aration of Pu2C3 is rather straightforward, thanks to the high thermodynamic stability of this phase. Th2C3 was successfully synthesized by Krupka and coworkers26,27 starting from arc-melted 57-67 at.% C alloys then sintered in a belt-type high pressure die under a pressure of 2.8-3.5 GPa between 1323 and 1623 K for 1 h.

The preparation of U2C3 is extremely difficult and it commonly requires a long (~1 day) annealing of a two-phase UC + UC2 metastable starting material in a narrow temperature range, between approximately 1720 and 1900 K. The annealing time can be reduced to a few minutes under particular conditions, for example, under high pressure or in a suitable atmo­sphere. Several ways of preparing U2C3 have been successfully explored. They can be regrouped in two main categories: those employing the ‘synthetic reaction’

UC + UC2 ! 2U2C3 [II]

and those based on the ‘decomposition reaction’

2UC2 ! U2C3 + C [III]

Several methods based on the synthetic reaction are available in the literature. For example, Matzke and Politis5 obtained U2C3 by annealing cast UC15 two-phase samples at 1720 K for 20 h under high vacuum. U2C3 was also obtained by Krupka28 at 1220 K under a pressure of 15 kbar for 2.75 min. In the light ofthis latter work, it seems difficult to believe that the application ofmechanical strain has no influ­ence on the synthesis of U2C3, as proposed by a few researchers.29,30 The work of Henney et al.31 showed that even a high content of oxygen impurities can have an important influence on the U2C3 synthesis rate. Starting from a UC158 sample with 2900 ppm of oxy­gen, these authors obtained almost pure U2C3 after annealing for 74 h at 1773 Kunder vacuum. The extra carbon reacted with oxygen to form CO and CO2, fostering the formation of the sesquicarbide.

Producing or quenching cubic fcc-KCN-like acti­nide dicarbides to room temperature is virtually impossible due to the martensitic nature of the cubic! tetragonal transformation and its extremely fast kinetics. Tetragonal dicarbides, on the other hand, are easily quenched even when they are not in a thermodynamically stable phase at room tem­perature (as in the case of a-UC2).

The rate of oxidation of PuC and ThC in air is much higher than that of UC and (Th, U)C and (U, Pu)C solid solutions, whereas it is much lower in sesquicarbides.

The oxidation of actinide carbides occurs some­times with the formation of flames (pyrophoricity), especially in samples with large specific surface (fine powders).

Actinide carbides tend to hydrolyze in water and even on exposure to laboratory air, where they exfo­liate, increase in weight, and produce final hydrolysis products.