MMR Low Temperature Measurement

• This is a vacuum system. Use gloves religiously.
• If you have used up the last of the gloves or the cotton swabs, go get more.
• Using up the last of the UHP nitrogen is defined as reaching 2000 PSI on the main tank gauge.
• All of the valves and manipulators have stops to indicate maximum range. Some of these stops are quite subtle. Do not apply excessive force to the valve controls or the manipulators.
• Clean up after yourself!
• Make sure you fill in log book; it helps in keeping track of nitrogen usage and possible recurrent clogging problems.
• Don't forget to turn off the K20 temperature controller and the microscope light when you're done with them.

Venting/Mounting your sample

1. Open the vent valve all the way.
2. Open the main tank valve on the industrial nitrogen cylinder. DO NOT adjust the tank regulator; it has already been adjusted to 40 psi. While this may seem like a low pressure for compressed gas, it's more than enough to pop the window off if you're not careful.
3. Use an allen wrench to slowly open the vent regulator valve for the argon adjacent to the vent valve. You should be able to hear the gas entering the system. Make sure to hold a gloved hand over the window as the positive pressure inside the chamber may unexpectedly pop the window off. (Ask Tim Wisleder if you don't believe this can happen.)
4. With nitrogen flowing into the chamber, remove the window by screwing in the nylon screws to push the window up.
5. Once you've removed the window, you can turn up the flow of argon to keep the chamber free of moisture.
6. Mount the sample:
   1. The sample stage is made of glass and is supported only at the entry point to the chamber. Do not apply pressure to the sample stage.
   2. Move the probes away from the stage.
   3. Remove old sample (if present):
      • Carefully remove sample with tweezers.
      • Use a cotton swab soaked in isopropanol to remove the thermal grease from the sample stage.
      • If desired, use the hot air gun on low setting for 10-15 seconds to dry the sample stage. Be sure to move the gun around continuously.
   4. Apply thermal grease to the stage using a cotton swab.
      • Use only Dow Corning Thermal Grease #340. This should be wrapped in a clean wipe in the drawer beneath the probe station. Please store the grease in this manner.
      • You only need a tiny amount to mount the sample to the stage. Excessive grease may get on the probe tips or on the front of your sample.
      • It's easiest to apply the grease with the wooden end of a cotton swab. There should be a supply of wooden end cotton swabs (as opposed to q-tips) in the drawer beneath the probe station, either in a bag or wrapped in a clean wipe, next to the thermal grease.
      • Do not get thermal grease on parts of the J-T refrigerator other than the gold rectangle.
      • Carefully place your sample on the stage. The gold rectangle is the only part that changes temperature; keep this in mind when you position your sample.

Evacuating

1. Place the window in the opening, making sure that the holes for the nylon screws ARE NOT aligned with the holes beneath the window.
2. Close the main tank valve on the industrial nitrogen cylinder.
3. After the pressure reading on the vent regulator valve gauge (next to the vent valve) goes to 0, close the vent regulator valve and the vent valve.
4. Make sure the turbo pump is set to low speed operation; this is important as we no longer have a house vacuum connection to rough out the pumping station. Pressing the low speed button toggles between the regular and low speed operation modes; the low speed mode is indicated by "LS" on pumping station display.
5. The window should seal; if the window does not seal, repeat the vent and evacuation procedures, realigning the window with the opening.

Purging the J-T Refrigerator

1. Make sure the high pressure nitrogen (HPN) tank regulator is closed.
2. Open the HPN main tank valve.
3. Set the HPN regulator to 500 PSI.
4. Record the tank pressure in the "before" column of the log book.
   
5. Purge the system for at least 3-5 minutes (20 minutes if the system has been clogging). If you're going to cool down to 80K it's advisable to purge the system for at least 10 minutes; if you're planning on doing some high temperature measurements before your low temperature measurements, 3-5 minutes is sufficient.

Using the System

1. Turn on the K20 temperature controller.
2. The temperature range of the current refrigerator is 80K to 730K.
   • Heating faster than 15K/minute can damage the refrigerator.
   • Most of the labview programs on Duke that control the K20 take into account this maximum heating rate. Some older programs, as well as those on lab controllers other than Duke, may not be corrected for this limitation. Those older programs were written for a different J-T refrigerator that had a less powerful heater element. Use of such programs may result in damage to the system (after which there will be significant damage to YOU). If you don't know if the program you want to use controls the rate of heating, ask.
   • Programs for the MMR are located in C:\Otherprg\MMR\
     
3. If you want to cool, increase the HPN to 1500-1800 PSI.
   • The higher the HPN pressure, the faster the cooling.
   • 1800 PSI is the maximum pressure that should be set; higher pressure may damage the etched channels of the refrigerator.
   • In general, it is difficult to probe devices at low temperature due to condensation. The best method is to move the probes to the desired device at room temperature, then decrease the temperature to the minimum, and take your measurements as you increase the sample temperature.
   • If the system fails to cool, it is most likely due to clogging of the etched channels. Clogging occurs when moisture present in the etched channels freezes, thereby impeding the flow of gas through the channels.
     • If the system is clogging and you want to do high temperature measurements as well as low temperature measurements, you might want to try doing the high temperature measurements first.
     • Two signs of clogging are an inability to cool down to 80K, and a drop in nitrogen flow out of the refrigerator.
     • The is a labview program, "clogging.vi", in the K20+4145 labview library that can guide you through the declogging procedure.
4. If you want to heat, leave the HPN at 500 PSI.

Venting after Using the System

1. Turn off the K20.
2. Record the HPN tank pressure in the "after" column of the log book.
   
3. Close the HPN main tank valve.
4. When the HPN regulator read 0, close it..
5. Turn off the turbo pump. Let it spin down completely.
6. Follow venting procedure above.
7. Remove sample and clean off the thermal grease with a cotton swab and isopropanol.
   • If you do not remove the thermal grease, it will harden and become difficult to remove.
   • Try not to hit the probes with the cotton swab.
   • Be gentle when cleaning thermal grease near the mylar foil. The mylar foil is very fragile, and it is crucial to achieving low temperatures.
   • Dry the sample stage with the heat gun on its low temperature setting.
8. Replace the window.
9. Turn on the turbo pump station at its low speed setting to evacuate the chamber and seal the window.
10. Clean up after yourself.
11. Fill in the log book.