Alloy, Curie point

The Curie point of the alloy is determined by the iron-to-nickel ratio an Fe Ni ratio of 50 50 was used to give a Curie point of 530°C. This pyrolysis temperature was chosen because it has been shown22 to give a balance between fragmentation from polysaccharides and protein fractions. Foils with Curie points of 300°C to 1000°C are commercially available (Figure 15.2). How-... 

Figure 15.2 Curie-point pyrolysis Temperature profile and Curie points of typical ferromagnetic alloys used.

Cobalt has the highest Curie point of any metal or alloy of cobalt. The Curie point is the temperature at which an element will lose its magnetism before it reaches its melting point. 

Cobalt s Curie point is 1,121°C, and its melting point is 1,495°C. About 25% of all cobalt mined in the world is used as an alloy with other metals. The most important is the alloy alnico, which consists of nickel, aluminum, and cobalt. Alnico is used to make powerful permanent magnets with many uses, such as CT, PET, and MRl medical instruments. It is also used for electroplating metals to give a fine surface that resists oxidation. 

Ehrenfest s concept of the discontinuities at the transition point was that the discontinuities were finite, similar to the discontinuities in the entropy and volume for first-order transitions. Only one second-order transition, that of superconductors in zero magnetic field, has been found which is of this type. The others, such as the transition between liquid helium-I and liquid helium-II, the Curie point, the order-disorder transition in some alloys, and transition in certain crystals due to rotational phenomena all have discontinuities that are large and may be infinite. Such discontinuities are particularly evident in the behavior of the heat capacity at constant pressure in the region of the transition temperature. The curve of the heat capacity as a function of the temperature has the general form of the Greek letter lambda and, hence, the points are called lambda points. Except for liquid helium, the effect of pressure on the transition temperature is very small. The behavior of systems at these second-order transitions is not completely known, and further thermodynamic treatment must be based on molecular and statistical concepts. These concepts are beyond the scope of this book, and no further discussion of second-order transitions is given. 

For the CDA experiments described below a PERKIN-ELMER TGS-1 system was used with a cylindrical furnace on alumina ceramic, 6 mm i.d., with bifilar Pt windings as sketched in Fig. lb. The sample, 4x4 mm and 1.5-3 mm thick, was suspended from a fused silica hang-down wire and rested on a fused silica loop below the rim of the furnace. A Ni electrode 3 x 5 mm was introduced sidewise above the furnace, parallel to the sample surface at a distance of <0.5 mm. The temperature (T) was calibrated by means of the Curie points of ferromagnetic alloys. During the runs the balance was... 

At a temperature below the Curie point TC1 it is plain from Eq. (3.7) that for concentrations nearer than a certain critical concentration the alloys will bo below their Curie points and will be in partly ordered phases, while for x less than this critical concentration they will be above their Curie points and will be in the disordered state. This is indicated in Fig. XVIII-7, where wo show G as a function of w for different values of x, at a temperature of 0.8 Tc. The critical concentration for this temperature is 0.277, as can be found at once from Eq. (3.7) it is noted in Fig. XVIII-7 that the curves for x = 0.1 and 0.2 definitely have their minima at w = 0, indicating complete disorder, while that for x = 0.3 is very flat at the center, and those for 0.4 and 0.5 definitely have minima for w 0, indicating a partly ordered state. Finally, in Fig. XVIII-8 we show G as a function of x, for different values of w, at this same tern-... 

A technique has been developed for data storage based on the magnetic behaviour of thin magnetizable Ln-alloy films. The fact that gadolinium has a curie point at room temperature and can be driven above this point by local heating is used. 

Table 4.2.1. Curie points of severai ferromagnetic alloys.

As seen in Table 4.2.1, temperatures in a wide range can be obtained using different compositions for the ferromagnetic alloy. However, the temperatures obtained with Curie point instruments cannot be varied continuously. 

The Curie point pyrolysers have several advantages when compared to other systems. The TRT is usually short and the heating rate is reproducible. The Teq temperature is accurately reproducible for the same alloy. The contact between the sample and the heated alloy is good, which assures that the heat transfer to the sample is rapid and uniform. On the other hand, the set temperatures can only be discrete and are limited to the values offered by different alloys. Even though the direct contact of the sample with the ferromagnetic alloy offers the advantage of a good heat transfer, it can be a... 

Different practical constructions of a Curie point pyrolyzer are commercially available. In these systems, the sample is put in direct contact with the ferromagnetic alloy, which is usually in the shape of a ribbon that can be folded over the sample forming a sample holder. The sample and its holder are maintained in a stream of inert gas in a similar way as for resistively heated filaments. The housing where the sample and its ferromagnetic holder are introduced is also heated to avoid the condensation of the pyrolysate but without decomposing the sample before pyrolysis. Autosample capabilities for Curie point pyrolyzers are also commercially available (e.g. DyChrom modelJPS-330) [11, 12]. 

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