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Post by Lake Shore Scott C on May 18, 2017 11:19:23 GMT -5
You're welcome. Good luck1
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Post by Lake Shore Scott C on May 18, 2017 11:18:49 GMT -5
I not aware of any other affect that would cause the slow thermal relaxation. It sounds like the chip should be well heat sunk, but I do wonder if something is going on with the leads. I agree that you wouldn't see sudden jumps in the voltage when the superconducting leads go normal due to the four lead measurement. If there is a convenient way to do so it may give a clue to what's happening by measuring the devices in a 2-lead measurement scheme. In the best case, the conversion from 4-lead to 2-lead would be done near the mixing chamber (cold plate or still?). In this scenario, you might be able to correlate the change in relaxation behavior in going from the 30-80 mK region to the above-100 mK region with a change in the behavior of the superconducting leads. Also, what other thermometer are you measuring the time response against? Thanks
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Post by Lake Shore Scott C on May 17, 2017 11:02:19 GMT -5
Thanks for your question. The mounting method depends on a few parameters includinig temperature range and ease of removal. Is this to be a permanent mounting? If so, you might consider an epoxy like Stycast 2850. If not permanent, then the question becomes how easy you need it to be to remove the sensor. Soldering requires heating the surface where it's mounted which isn't always ideal, but you can use IMI-7031 varnish. You'll need to use a solvent to dissolve it when you want to remove it. The spring clamp is also a useful way to mount it, although I would also use Apiezon N-grease to fill voids between the sensor and mounting surface. That has the advantage that the grease will harden as temperature drops and help hold it in place while giving a better thermal connection as compared to just a spring clamp. As long as the spring clamp captures the SD package to prevent it from moving, it should be a perfectly good way to mount the sensor. With regard to the solder question, most of the lower melting point solders aren't as strong as what we've come to expect from 60Sn/40Pb or 63Sn/37Pb. There are a number of solders that can be used. I would probably use indium solder with a melting point of 156.7 degC. It's had a long history of use in cryogenics and should be reliable. A company with a wide selection of solder choices is www.aimsolder.com and they can send you a list of solders in Excel spreadsheet form allowing you to sort by melting point. Good luck and let us know if you have further questions.
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Post by Lake Shore Scott C on May 11, 2017 10:48:49 GMT -5
I have just a couple of quick questions to help answer your question. First, you mention that "... then the relaxation of the chip back ot the temperature of the bath ..." - is the chip mounted directly directly in the He3-He4 mix in the mixing chamber? If not, is the chip mounted externally against the mixing chamber in vacuum? If mounted externally in vacuum, how is the chip heat sunk against the mixing chamber? The bare chip would need an insulating layer to keep it from shorting out when mounted against any metal part of the DR cryostat, and I would expect very little heat flow / thermal connection through the superconducting NbTi lead attachments at those temperatures. Is it possible that the chip isn't we'll heat sunk and your seeing the long thermal response as a result of limited heat flow through the superconducting leads?
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