Superconducting materials magnetic fields

Based on their difference in reacting to magnetic flux, superconducting material can be differentiated into Type-I and Type-II. The Type-11 superconductors are usually associated with intermetallic compounds instead of elements and their superconductivity is not easily affected even with a high magnetic field. 

Fig. 8. Typical magnetization curve for a high-field superconducting material.

Three important characteristics of the superconducting state are the critical temperature, the critical magnetic field, and the critical current. These parameters can be varied by using different materials or giving them special metallurgical treatments. 

In this superconducting state, the electrical resistivity of the material becomes zero, and its thermal properties change. Tc is a function of the material as well as of its purity and of the applied magnetic field. 

In particular, if the magnetic field is strong enough, the material does not enter the superconducting state. The latter property is shown in Fig. 3.4(a) where the specific heat of A1 was measured [17] in no-field and in a moderate field. 

We can conclude that the new behaviour of the superconducting material is due to a new state for the electrons in fact, at the critical temperature, there is a jump of the electronic specific heat. In no external magnetic field, it is a second-order transition, which does not involve latent heat. 

In Section 3.10, we have seen that the thermal conductivity of a superconducting metal can become very small at temperatures well below the transition temperature Tc (about 1/10 of Tc) it can be orders of magnitude smaller than the thermal conductivity of the same material in the normal state. Some metals can be easily switched from the superconducting to the normal state by applying a moderate magnetic field (10 2-10 1 T). 

One device is made up of samples of superconducting materials put inside a transformer a low-frequency small current in the primary creates a weak magnetic field. 

In superconducting transitions, the purity of materials and the shielding from magnetic fields are fundamental to obtain steep transitions (see Figs 8.6-8.8). 

Figure 8.6 shows the definition of transition temperature Tc and transition width Wc. To remark how the purity of the material and the magnetic field influence the two parameters, we will report hereafter the measurements of the superconducting transition of titanium samples of different purity and in the presence of different magnetic fields. The transitions of four Ti samples, whose characteristics are reported in Table 8.12, are shown in Fig. 8.7 where a change of only 0.2% in purity dramatically moves both the transition... 

To overcome these practical problems, research aimed at the preparation of new materials exhibiting superconductivity at higher and higher temperatures and able to sustain this state even in the presence of strong magnetic fields (or, also in the presence of high electric currents). 

The Meissner Effect and Levitation. Besides the absence of electrical resistance, a superconducting material is characterized by perfect diamagnetism. The exclusion of magnetic field lines from a material when it passes from a normal state to a superconducting state is shown schematically in Figure 3. 

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