Does a type I superconductor have to be single crystal?

[zhanhai]

Does a type I superconductor has to be single crystal?

If not, is there any change in its Tc when it becomes polycrystal?

Thanks.

 

[ModestyKing]

(Note: I am not an expert, just a student)

No, a type 1 superconductor does not necessarily have to be a single crystal. Being poly-crystalline means there are grain boundaries, which can count as defects in the lattice. To electrons, then, they look like scattering sites. However, according to BCS theory (accepted theory for type 1 superconductors), superconductivity occurs when electrons form Cooper pairs - they have a slight binding energy because as one electron distorts the crystal lattice, this slight distortion in the nuclei's positions is favorable to the next electron passing nearby. Because of this binding energy, it is much less likely that when 1 electron wants to scatter (like at a grain boundary), it will. Because this would require enough energy to break up the Cooper pair.

So no, a type 1 superconductor does not necessarily have to be a single crystal. Of course, it helps. I don't know how being poly-crystalline would affect the critical temperature, but my gut feeling says the Tc would slightly decrease for polycrystalline materials, if at all.

 

[f95toli]

No, there is no need for it to be a single crystal. In fact, we rarely work with single crystals when dealing with type I SC since these are generally metals such as aluminium.
A SC that is polycrystalline can have slightly different properties than a single crystal. However, there is no easy answer to just how different it will be since this will depend on which SC it is and also the structure (size and orientation of the grains etc).
Note also that it is by no means certain that Tc will go down; the Tc of e.g. aluminium tends to go UP when you introduce strain and impurities (this is why thin films of aluminium tends to have a Tc of about 1.6K wheras the bulk value is 1.2K).