Thermomagnetic curves

Figure 2. The thermomagnetic curve of cyclic heating of alloy Fe82Si2Bl 6.

Thermomagnetic curves for all samples were measured at 3 kOe. The temperature gradients of the magnetic moment are less than one per cent for all samples. The temperature dependence of magnetization is characterized by the positive curvature. This fact is specifying ferromagnetic ordering of the particles. 

On this basis, a representation of the magnetisation (that can be obtained in general with magnetometers or can be considered proportional to the ferromagnetic resonance signal obtained in ESR spectrometers) vs. temperature yields thermomagnetic curves as a function of the particle volume.11 From comparison of experimental data with such curves, an estimation of the mean particle size can be obtained. Size distributions can... 

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). 

One other feature of thermomagnetic curves will be mentioned. Figure 37 shows I versus T in the neighborhood of the Curie point. It will be seen that I does not drop to zero at Tc, but rather tails off rather slowly as T is raised. The reason for this is that most substances which are ferromagnetic at low temperatures follow the Curie-Weiss law above Tc. Near Tc the term T + A) approaches zero, so that x becomes very large and may become somewhat dependent on H. Somewhat arbitrarily Tc is generally taken as the inflection point as shown in Fig. 37. This procedure proves satisfactory in practice. 

Fig. 38. Thermomagnetic curve for Fischer-Tropsch iron catalyst, / original, II...

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. 

It must be remembered that chemical reactions in the solid state may take place while the thermomagnetic curves are being obtained This is... 

Fig. 41. Thermomagnetic curve showing reactions proceeding as the temperature is...

Fig. 42. Thermomagnetic curves for I nickel supported on magnesia, and 11 for reduced nickel ammonium molybdate. The ordinate for the former is reduced, and the units are arbitrary.

X-ray, chemical analysis, and magnetic measurements had all been made on the same sample, thus definitely relating the various Curie points to definite compounds. The thermomagnetic curves involve some complexities which are described below. 

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°. 

Fig. 44. Thermomagnetic curves, I for Hagg carbide, II same after beating to 580. ...

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. 

Fio. 45. Thermomagnetic curves, for hexagonal iron carbide, I with rising temperature, II with falling and subsequent temperature cycles. 

Figure 49 shows thermomagnetic curves for fresh catalyst, partially deactivated catalyst, and completely deactivated catalyst. The par-... 

Fig. 49. Thermomagnetic curves for nickel supported on copper for A active, B partially deactivated, and C inactive samples.

It has been shown by Marian and others (Marian, 57 Ross, 58) that for solid solutions of copper in nickel, both Curie point and specific magnetization are linear with respect to concentration. Marian s data for the Curie points are shown in Fig. 50. The thermomagnetic curve for a homogeneous solid solution has the same shape and abrupt Curie point shown by a pure ferromagnetic substance except that the specific magnetization (tr ) at any given temperature is lower, and the... 

Curie point is also lower. But for an alloy mixture consisting of a one-phase continuous series of solid solutions the thermomagnetic curve may be quite different. Such alloys may be said to be nonequilibrium solid solutions, although the metallurgists refer to such a condition as a micro-inhomogeneity. The form and usefulness of thermomagnetic... 

Fio. 51. Thermomagnetic curves for copper-nickel powder mixture as a function of... 

Returning now to the thermomagnetic curves in Fig. 49, one sees how to interpret the data in terms of catalyst structure. At the temperature of the hydrogenation reaction, 175°, the fresh catalyst was only slightly ferromagnetic, and the partially deactivated ones were not ferromagnetic at all. It is clear that the active mass need not be ferromagnetic at the temperature of the reaction. 

Fig. 62. The thermomagnetic curves of a single crystal of non-stoichiometric YFejO. The measurement was carried out with increasing temperature. (Sugihara et al. 1978.)...

Fig. 63. The thermomagnetic curves of polycrystalline InFCjO,. In an external field of 1 kOe. (Gerardin et al. 1981.)...

Fig. 70. The thermomagnetic curves of polycrystalline LuFe204 showing the field heating effect, (a) in an external field of 4.5 kOe (b) in an external field of 105 Oe. In both cases, the upper curves are for a field-cooled specimen whereas lower...

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