46
c
HAPTER
2:
Cooling System Design and Temperature Control
Model 372 AC Resistance Bridge and Temperature Controller
used or the parts will break apart, potentially damaging them. The thermal expansion
or contraction of rigid clamps or holders could crush fragile samples or sensors that
do not have the same coefficient. Thermal conductivity is a property of materials that
can change with temperature. Do not assume that materials that work well at room
temperature will do the same job at low temperatures.
2.10.3.2 Sensor Location
Finding a good place to mount a sensor in an already crowded cryostat is never easy.
There are fewer problems if the entire load and sample holder are at the same tem-
perature, unfortunately, this not the case in many systems. Temperature gradients
(differences in temperature) exist because there is seldom perfect balance between
the cooling source and heat sources. Even in a well-controlled system, unwanted heat
sources like thermal radiation and heat conducting through mounting structures can
cause gradients. For best accuracy, sensors should be positioned near the sample, so
that little or no heat flows between the sample and sensor. This may not, however, be
the best location for temperature control as discussed below.
2.10.3.3 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. Metals that super conduct
tend to be better thermal conductors than normal and poor thermal conductors
below their transition temperature. Non-metallic, electrically insulating materials
like alumina oxide and similar ceramics have good thermal conductivity, while G-10
epoxy-impregnated fiberglass does not. Sensor leads should have poor thermal con-
ductivity. Sensor packages, cooling loads, and sample holders should have good ther-
mal conductivity to reduce temperature gradients. Surprisingly, the connections
between thermally conductive mounting surfaces often have very poor thermal con-
ductivity.
2.10.3.4 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 con-
ductivity. They must also withstand the environmental extremes. Indium foil (when
not super conducting) and cryogenic grease are good examples.
2.10.3.5 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 overheat or mechanically stress sensor packages. Less perma-
nent mountings require some pressure to hold the sensor to its mounting surface.
Pressure greatly improves the thermal resistance of joints and reduces thermal gradi-
ents. A spring clamp is recommended so that different rates of thermal expansion do
not increase or decrease pressure with temperature change.
2.10.3.6 Lead Wire
Different types of sensors come with different types and lengths of electrical leads. In
general a significant length of lead wire must be added to the sensor for proper heat
sinking and connecting to a bulk head connector at the vacuum boundary. The lead
wire must be an electrical conductor, but should not be a good thermal conductor, or
heat will transfer down the leads and change the temperature reading of the sensor.