Hexagonal carbide

Carbides of iron are thermodynamically unstable with respect to decomposition to iron and carbon. When hexagonal carbide is heated in an inert atmosphere the following sequence of reactions is observed ... 

Hexagonal carbide may be prepared by prolonged treatment of reduced catalysts with carbon monoxide at temperatures below 200°C. ... 

The hexagonal carbide (e-Fe2C) forms initially when carbiding occurs below 240°C using either CO + H2 or hydrocarbon gas. This converts to x Fe5C2 upon further reaction with C or if the oxide phase should form. The use of hydrocarbon gases (butane, propane or pentane) avoids the presence of oxygen as well as the rapid T rise found when CO is used. In addition, free carbon can be avoided. ... 

One hexagonal carbide, NijC, is reported , but the structure is isomorphous with e-FcjC rather than with C03C or FejC. This material must be prepared by the action of CO on the metal at T near 350°C It does not result from the direct reaction between the elements, suggesting an oxycarbide. It decomposes into the elements above 430°C in vacuum. ... 

In the case of iron catalysts, x-ray and thermomagnetic investigations confirm the work of Pichler and Merkel and show that the Fe2C with the Curie point 265°C. of Pichler and Merkel is identical with Hagg s carbide. Hofer, Cohn, and Peebles found the inflection point of the thermomagnetic curve at 247°C. The Fe2C with the Curie point 380°C. of Pichler and Merkel seems identical to a hexagonal carbide identified independently in the research laboratories of I. G. Farbenindustrie by work of Halle and Herbst (90). 

Halle and Herbst obtained a hexagonal carbide by carburization of iron-copper catalysts, and later also by carburization of copper-free catalysts (reduction and carburization at low temperatures). The x-ray pattern is not identical to that described by Hagg. On the basis of their x-ray investigations Hofer, Cohn, and Peebles believe that the carbide of Halle and Herbst is identical to the Fe2C carbide with a Curie point at 380°C. of Pichler and Merkel (see Sec. III.4.d). 

Toward the end of 1995, we identified a new class of solids best described as machinable, thermodynamically stable polycrystalline nanolaminates (Fig. 1.4a). These solids are ternary layered hexagonal carbides and nitrides with the general formula, M + AX , where = 1 to 3, M is an early transition metal, A is an A-group element (mostly IlIA and IVA) and X is C and or... 

A thermomagnetic curve for hexagonal iron carbide is shown in Fig. 45-1. This shows two Curie points, one at 247° due to Hagg carbide, and the other at 380° apparently due to hexagonal carbide. The appearance of the 247° Curie point is surprising because x-ray lines for carbide were obtained only with considerable difficulty, but this merely illustrates the usefulness of the magnetic method for detecting a substance which neither x-ray nor chemical analysis can readily reveal. 

From these results it is possible to bring some order out of a rather confused situation with respect to the several iron carbides. As pointed out by Hofer, the hexagonal carbide is identical with the carbide described by Pichler and Merkel as having a Curie point at 265°. This carbide can also be identified with the cubic ferric oxide saturated with potassium oxide of Lefebvre and LeClerc (Refs. 51-53). 

---