Greetings
A year ago, I had prepared (Bi,As)-2212 superconducting cuprate samples, sintered at 840 degrees and the XRD showed a good phase formation but the resistivity displayed a Tc 10 K lower than expected. For curiosity I had chosen a pellet of the pure sample and one at x=0.1 of As. Those pellets were placed in a tube furnace for 2 hours under Argon flow at T=845. This was done in the hopes of somewhat reducing Oxygen content (as EDX results were going to be late for logistical reasons). When I removed them I found that they had completely melted and I'm still baffled till this day.
Some info that might be helpful:
Before the press, the precursor powder was ball milled at a ratio of 10:1 for 30 minutes and then pressed at around 1 GPa, then sintered at T=840 for 50 hours.
Calculations using the Scherrer equation from XRD showed that the average crystalline size was around 32 nm.
I must've discarded the notes regarding the flow rate due to the method being unhelpful, so I don't have the flow rate for the Argon to share with you.
Best regards
Bi-2212 materials are very robust in the form of single crystal. I have measured the electrical resistivity and magnetic measurements MT, MH after the 3 years of synthesis and results were found to be almost similar. Here, an important point to be noticed is in case of single crystal sample can be cleaved and fresh surface is obtainable. However, in case of polycrystal it is difficult. I would suggest to store the samples in inert atmosphere. If your sample is really have the chance to absorb the oxygen, then just heating and removal of oxygen is quite risky in atmospheric conditions. Scrubbing or fracture process can help you to find the fresh material, only, when the density of the prepared material is quite high (atleast 98%). Else porous materials have very high chance to absorb oxygen as it is light element and available in atmosphere. For the small atmoic mass number element, do not believe much the SEM-edx results as the detector used in SEM is not good enough to provide the results within 2% accuracy. I would suggest use EPMA, in which single crystal detector is used which can provide much accurate results, especially, for e.g. oxygen, boron, nitrogen, Lithium element content in the compound.
You are right. Research in this area suggests that re-experimentation is recommended. Good luck with your work.
Thank you Dr Saurabh Singh for such an elaborate answer and one that I will keep in my mind in future projects.
The main dilemma that I had was regarding the fact that the samples had melted completely after 2 hours of sintering with Argon flow at T=845 even though they were prepared at T=840 for 50 hours.
Do you think what happened was due to the extra pressure that the Argon created or is it possible that the flow removed plenty of Oxygen content, causing a different melting point to manifest or the structure to collapse (or both).
I would love to hear your opinion on that.
Best regards.
As you mentioned about argon flow effect on melting. I do not think about melting cause due to the argon as it is inert gas. Melting might be possible to formation of some other phase (impurity, contamination, composite). I recommend you to make at least XRD or chemical composition measurements before doing annealing or any measurements. As a back characterization will help you to analyze and plan the next steps. If you really want to explore the reason, I would suggest to make high temperature TGA-DTA measurements or DSC measurements. You can easily analyze the phase transformation or decomposition effect. If xrd is freely access to you place, then make xrd at each steps. You can easily probe if structure phase transformation occurs, or any other phase formation occurs in the process you expose to the sample. The best and simple solution is take xrd, measurements of density, keep the samples in desicator. And take xrd at each steps to see if something changes. Good luck.
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