Lake Shore Model 340 Temperature Controller User’s Manual
Cooling System Design
2-5
2.3.3 Thermal
Conductivity
The ability of heat to flow through a material is called thermal conductivity. Good thermal conductivity is
important in any part of a cryogenic system that is intended to be the same temperature. Copper and
aluminum are examples of metals that have good thermal conductivity, while stainless steel does not.
Non-metallic, electrically-insulating materials like alumina oxide and similar ceramics have good thermal
conductivity, while G-10 epoxy-impregnated fiberglass does not. Sensor packages, cooling loads, and sample
holders should have good thermal conductivity to reduce temperature gradients. Surprisingly, the connections
between thermally conductive mounting surfaces often have very poor thermal conductivity.
Drawing
Not To Scale
To Room
Temperature
Refrigerator
Expander
Refrigerator
Second
Stage
Dental
Floss
Tie-Down
Thermal
Anchor
(Bobbin)
Thermal
Anchor
(Bobbin)
Radiation Shield
Sensor
Cold Stage and
Sample Holder
Optical Window
(If Required)
Cryogenic Tape
Cryogenic Wire
(small
diameter,
large AWG)
-or-
Vacuum Shroud
Vacuum Space
Heater
(wiring not shown
for clarity)
340-2-2.eps
Figure 2-2. Typical Sensor Installation In A Mechanical Refrigerator
2.3.4 Contact
Area
Thermal contact area greatly affects thermal conduction because a larger area has more opportunity to
transfer heat. Even when the size of a sensor package is fixed, thermal contact area can be improved with the
use of a gasket material. A soft gasket material forms into the rough mating surface to increase the area of
the two surfaces that are in contact. Good gasket materials are soft, thin, and have good thermal conductivity.
They must also withstand the environmental extremes. Indium foil and cryogenic grease are good examples.
2.3.5 Contact
Pressure
When sensors are permanently mounted, the solder or epoxy used to hold the sensor act as both gasket and
adhesive. Permanent mounting is not a good solution for everyone because it limits flexibility and can
potentially damage sensors. Much care should be taken not to over heat or mechanically stress sensor
packages. Less permanent mountings require some pressure to hold the sensor to its mounting surface.
Pressure greatly improves the action of gasket material to increase thermal conductivity and reduce thermal
gradients. A spring clamp is recommended so that different rates of thermal expansion do not increase or
decrease pressure with temperature change.