Greases and Adhesives Used in the Cryostation
Small amounts of Apiezon N-grease and L-grease, as well as GE Varnish, are included in the accessory kit that comes with each system.
N-grease is rated for vacuum and cryogenic temperatures. It is used in the sample chamber for metal-to-metal contact, or holding the small and thin low working distance windows in place. N-grease only needs be applied where thermal transfer is needed, for instance, it is used between the platform and the bottom of the sample mount.
Image: Courtesy Apiezon.com
L-grease is for ultra-high vacuum, and is ideal for o-rings. The locations of the o-rings in the sample space are on the top and bottom of the housing, and there is also an o-ring below the PCB. There are o-rings in each window. Check that the o-rings on the housing are greased, they may need to be re-greased after 10-15 uses.
Grease Application Technique
Spread the grease evenly around o-ring or on metal surface. Dab in several spots and then blend in, so an even amount of grease is distributed on the surface. You do not need much grease, just enough to create a shine to the surface of the o-ring, as too much could hinder the vacuum seal or the thermal transfer through the two metal surfaces.
To maintain the integrity of the o-rings, place the housing on the flattened corner of the housing when it is not on the sample chamber and always inspect for any debris before placing on the sample chamber.
Do not interchange the greases! The N-grease should only be used for metal-metal or metal-glass interfaces. L-grease should only be used on the o-rings.
GE Varnish (VGE) is an adhesive used for cryogenic applications. VGE is typically used when lagging thermometers or other wiring. VGE can be cured for several hours in the open air, or can be placed in an oven at 60°C for about 10 minutes. A heat lamp can also be used to cure VGE in the sample space, the time to cure is about 30 minutes. It is important to not pull vacuum on the Cryostation before the VGE is cured.
VGE Application Technique
For good thermal contact between the metal and sample or thermometer it is important to start with a clean surface. A thin layer of VGE can be applied and the object to be mounted should be in place within 10 seconds of applying the VGE. Slight pressure should be applied after mounting, and while it’s best to keep pressure while the VGE cures, it isn’t essential. Note that acetone or isopropyl alcohol will remove VGE. Acetone can also be used to thin VGE if only a very small amount of VGE is needed. For example, acetone is mixed with VGE before placing it on the bottom of the thermometer to ensure that the thermometer is in good contact with the metal surface below. If the VGE is too thick under the thermometer there will not be good thermal contact between the thermometer and the surface and the temperature reading will be incorrect.
Sample mounting can be done with VGE, N-grease, or any other adhesive/grease that is rated for vacuum and cryogenic temperatures.
The low working distance vacuum windows are set with a two part epoxy. This window cannot be removed and reset, as a vacuum seal will not be achieved.