YBCO critical temperature not right

donpeters

In doing some research using a basic superconducting kit with a susceptibility probe, we set out to find the critical temperature for the YBCO compound.

In our first attempts, we were only able to achieve a critical temperature of 78.8 K, and the actual value is said to be between 90 and 93 K.

We then tried applying different currents during the experiment and found that our critical temperature changed a little bit and got a little closer to the expected value, but we were still only able to achieve between 80 and 84 K for our critical temperature.

I feel like we are far off of the expected value, any insight or reasons as to why this may be?

Edgardo

1) I found a link here, maybe it helps:
http://www.mrsec.psu.edu/education/t...BCOproject.pdf
http://www.virtualflint.com/summer01/psu/ybco.html

Mr. Flint mentions in his report that he measured a lower critical temperature because the temperature sensor adapted faster to the lower temperature than the YBCO pellet (see page 9).
I don't know if this also applies in your setup.

2) The way you heat the chemicals in the oven seems to be important (see page 3 for a temperature-time diagram)

Another article (unfortunately I only found a German vesion) here shows a temperature-time diagram for the oven (see page 5). The diagram is important because it influences how much oxygen is in the "structure" (see page 4).
According to the article you heat the chemicals in the oven to 950°C in a specified manner (°C per second) and get

[tex]Y Ba_2 Cu_3 O_{7-x}[/tex]

with x = 1. But the resulting YBCO with x = 1 is still not superconducting.
The further procedure of heating at 500°C (see the temperature-time diagram on page 5) changes x to approximately 0 because oxygen is absorbed.
For x = 0, YBCO has a critical temperature of 90K. On page 4 you can read that x = 0.25 results in a critical temperature of 60K.

Note: I am not an expert and I have never experimented with superconductors.

seycyrus

I believe I might be of some help with your problem, but I'll need to know some more details about your processing procedure, such as calcination and annealing conditions.

Note that genrally speaking, the transition width for SC is "wider" for a susceptibility measurement as compared to a resistivity measurement.

Also note, that the critical temperature generally gets lower as you apply a current, so that is a bit puzzling as well.

ZapperZ

Did you check this by measuring the resistivity (your statement on "applying different currents" is vague and I'm not sure if you measurement the resistance here)? It would be worth while comparing the susceptibility measurement with the resistivity measurement - they should coincide very closely. If they don't, then my guess is that the susceptibility measurement isn't done accurately.

And to address what Edgardo said, most YBCO samples that are widely distributed at optimally-doped. This means that the Tc value is the highest possible. Underdoped and overdopped samples tend to "evolve" towards the optimally doped level over time. So while it is possible, I find it rather unlikely that this sample of YBCO is not at or near optimal doping.

Zz.