Curie carbides

Cementite, the term for iron carbide in steel, is the form in which carbon appears in steels. It has the formula Fe C, and thus consists of 6.67 wt % carbon and the balance iron. Cementite is very hard and britde. As the hardest constituent of plain carbon steel, it scratches glass and feldspar, but not quart2. It exhibits about two-thirds the induction of pure iron in a strong magnetic field, but has a much lower Curie temperature. 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

Figure 1. Magnetic effect in the Curie point of the -Fe3C carbide.

Methane-14 was prepared (Ciranni and Guarino, 1966) from aluminium carbide and pure T2O. The crude CT4, specific activity of 116,500 Curies per mole, was immediately diluted with a large excess of CH4 and subjected to a rigorous purification, including a preparative gas-chromatographic separation over a special capillary column that allows (Bruner and Cartoni, 1965) the complete resolution of the four tritiated methanes, from CH3T to CT4. The final sample, whose isotopic purity is illustrated in Fig. 5, was further diluted with CH4 to a specific activity of 0-2 Curies per mole to carry out the decay experiments. Nefedov et al. (1968) used CT4 prepared by essentially the same procedure in their study of the reactions of methyl ions in hydroxylic compounds. 

Pichler and Merkel (24) investigated the composition of iron catalysts at various stages of pretreatment and synthesis by chemical and thermo-magnetic analysis. Copper-free iron catalysts, carburized at 325°C. before medium-pressure synthesis, were virtually completely transformed to a ferromagnetic higher iron carbide with a Curie point of 265°C., whose formula corresponded to approximately Fe2C. 

Carburization of copper promoted (20% Cu) iron catalysts at 220-230°C. yielded not only the 265°C. Curie point carbide, but also a second ferromagnetic iron carbide whose Curie point was 380°C. 

At 205°C. iron was carburized largely to the 380°C. Curie point carbide. Equilibrium conditions were established between carbide and oxide. The higher carbides formed during carburization were very stable against hydrogen at synthesis conditions. 

Lefebvre and LeClerc (36) carried out thermodynamic studies on catalysts of the Fischer-Tropsch synthesis. They drew attention to the significance of the specific Curie points, of various compounds, for their activity or inactivity as catalysts. They assumed that the active catalysts were cubic iron oxide and hexagonal nickel. Pichler and Merkel (37) found that the Curie point attributed by Lefebvre and LeClerc to cubic iron oxide is actually the Curie point of one special form of Fe2C. The hexagonal nickel seems to be actually a nickel carbide. 

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). 

Curve II shows the same sample after having been heated to 800° in a stream of nitrogen. The Curie point is no less well defined, but has fallen to about 205° which is near the Curie point for cementite. The Hagg carbide has clearly undergone a reaction owing to its instability at high temperatures. 

When two or more substances are present in a mixture, their thermomagnetic curves are additive, as shown in Fig. 39. The resolution of these curves depends on the amounts of substance present, their specific magnetizations, and the spacing of their Curie points. Figure 40 shows one of the Pichler-Merkel catalysts in which two carbides are clearly shown. 

Figure 44-1 shows the thermomagnetic curve for the Hagg carbide, care having been taken not to raise the temperature above 300°. The reproductibility of this curve, with falling or rising temperature, is excellent, and the Curie point is clearly defined as 247° 3°. 

The sample of Hagg carbide was now heated at 580° for 2 hours, after which curve II Fig. 44 was obtained. The product, which has a Curie point of about 208°, appears to be even more magnetically homogeneous than the sample before heating. The Curie point agrees satisfactorily with that for cementite, and the x-ray diffraction pattern confirms this analysis. The reaction may be written... 

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). 

The results show that the magnetic susceptibility of these carbides agrees well with a Curie-Weiss law of the form X = Xq + (C/T — 0 ) above the temperature T. The only exception is samarium hypocarbide, for which a deviation from this law has been observed. This type of deviation also occurs in SmC2 and has been attributed to Van Vleck paramagnetism. 

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