Van der Pauw sample preparation errors

[Metallus]

Hello,
I'm carrying out electrical resistivity tests with the Van der Pauw method on a round sample (2 mm thick disk) of a doped ceramic but sometimes I have issues in the mounting of the sample.

I'm using a 4-probe tube where each of the 4 wires contacts with the "corners" of the sample. To keep everything together, the sample is mounted on a sample holder made of insulation material that keeps the wire in contact with it. Here's a pic of the set-up, profile view and top view

Sometimes when I attempt to pass current in any of the 8 configurations (reciprocal measurements and polarity inversion measurements), I get infinite resistivity. Is it due to the wires not being perfectly in contact with the sample? Why does it work in some configuration but give infinite resistivity in the other? And why sometimes the signal remains 0? Does it mean that it's short circuiting somewhere (as if the wires were touching)?

What am I doing wrong exactly? I made it work before, so I guess I'm just screwing up in the preparation of the sample somewhere, but I don't know where exactly, nor I know what could be the source of error. Let me know if you need additional details, as I am new to this technique.

Thanks

 

[Tom.G]

Sure sounds like a poor mechanical connection between the wires and the sample. Perhaps an oxide build-up breaking electrical contact or a piece of dirt in the contact area. The next time it happens, try moving one wire at a time to localize the problem. Increased contact pressure between the wires and sample is likely to help. A higher test voltage may break through a thin oxide layer. If you are using a constant current excitation, redesign or replace it with a higher compliance voltage (open circuit voltage). Make sure both the sample and the electrical connectors to it are freshly cleaned.

Please let us know what you find / how you solve the problem.

 

[Metallus]

Sure sounds like a poor mechanical connection between the wires and the sample. Perhaps an oxide build-up breaking electrical contact or a piece of dirt in the contact area. The next time it happens, try moving one wire at a time to localize the problem. Increased contact pressure between the wires and sample is likely to help. A higher test voltage may break through a thin oxide layer. If you are using a constant current excitation, redesign or replace it with a higher compliance voltage (open circuit voltage). Make sure both the sample and the electrical connectors to it are freshly cleaned.

Please let us know what you find / how you solve the problem.

Yes, it was indeed a fault in the contact between the wire and the sample. One of the wire was loose (sliding easily in the hole of the sample holder and not properly contacting with the sample). I solved this issue by slightly bending the end of the wires inwards, so as to guarantee at least a contact on the edge

Thanks for your input

 

[Lord Jestocost]

Pure mechanical contacts between metallic wires and ceramic samples are often unreliable. I generally coat the areas of the sample which finally make the contacts with the wires with thin stripes of either conducting silver, gold or platinum paste, depending on the temperature and atmosphere used during the measurements.