CRYOGENICS

MSN 506/Phys 580

CRYOGENICS

• Why low temperatures?• Heat Transfer• Behaviour of materials at low temperatures• Monitoring temperature• Refrigeration

Why do we need low temperatures ?•• PhysicsPhysics

– Reduced thermal energy kBT allows observation of quantum effects in large structures

– Quantum effects at lower frequencies (hw ~ kT)– Reduced carrier dynamics (scattering, generation)– Better detection performance in certain sensors– Reduced thermal fluctuations and noise– Effects that rely on low temperatures such as

superconductivity are observable– Superconducting magnets

•• BiologyBiology– Sample preservation– Certain techniques for studying otherwise elastic

samples

Basic refrigeration• Used in daily life

– Household refrigeration– Industrial refrigeration

Reverse heat engine

Heat Capacity• Amount of heat given to the material to

raise its temperature under sepcified conditions

Q = m c ΔT Joule/g/KSpecific heat capacity

Dependent on temperature for solids

Dependent on pressure and temperature for gases

Heat conduction

TH

QTL

VH VLI

R

Equivalent circuit

capacitor

Heat conduction• Multiple Physical Mechanisms

– Conduction– Convection– Radiation

• Conduction– Solids

• Convection– Fluids (gases and liquids)

• Radiation– All materials

Thermal conductivity• thermal conductivity = heat flow rate ×

distance / (area × temperature difference)

Unit : W/m/K

Glass 1.1 Aluminium 220Stainless steel 15Diamond 2000Silver 400Copper 385Air (STP) 0.024 Temperature dependent

Thermal conductivity

15 mm plate separation at 200 K

Vacuum is a good insulatorLow molecular mass gases are good conductors of heat

Thermal resistance due to contact

Thermal circuits

sample

Heat Load from experiment(electronics, lasers etc.)

Cooling Power

Heat load due to imperfect insulation or vacuum,And conduction by Radiation

Mechanical support acts as a thermal resistor

Radiation

• Stefan-Boltzman Law

at 100 K the energy flux density is 5.67 W/m2, at 1000 K 56,700 W/m2

May become important only at extreme low temperaturesOr may affect the hold time of cryogenic storage dewars

Materials at low temperatures• Reduced heat capacity

• Reduced heat condcutivity (generally) due to reduction of phonon density

• Problems with electrical contacts (Lead becomes a bad thermal conductor at cryogenic temperatures, silver based solder is preferred)

• Stainless steel is a bad thermal conductor, this can be used to advantage. Use stainless steel core coaxial cables for RF signals.

Monitoring Temperature

• Thermistors• Thermocouples• Diodes

Monitoring Temperature• Thermistors

Electrical Resistance is affected in different dominant ways in metals and semiconductors

Increasing temperature increases scattering which tends to increase resistance

For semiconductors however, carrier density also depends on temperature. At low temperature there are less carriers and resistance tends to incresed.

Monitoring Temperature• Metals

Monitoring Temperature• Non-Metals

Monitoring Temperature

Monitoring Temperature

Monitoring Temperature

Monitoring TemperatureThermocouples work better at high temperatures (Ovens)

Two wires of different materials made into junctions

Monitoring TemperatureA variety of thermocouples are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research, etc.).Type K (Chromel (Ni-Cr alloy) / Alumel (Ni-Al alloy))

The "general purpose" thermocouple. It is low cost and, owing to its popularity, it is available in a wide variety of probes. They are available in the −200 °C to +1200 °C range. The type K was specified at a time when metallurgy was nowhere near as advanced as today and consequently characteristics vary considerably between examples. There is another problem in that one of the consituent metals is magnetic (Nickel). The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354°C. Sensitivity is approximately 41 µV/°C.

Type E (Chromel / Constantan (Cu-Ni alloy))Type E has a high output (68 µV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that it is non-magnetic.

Type J (Iron / Constantan)Limited range (−40 to +750 °C) makes type J less popular than type K. The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 °C as an abrupt magnetic transformation causes permanent decalibration. Type J's have a sensitivity of ~52 µV/°C

Type N (Nicrosil (Ni-Cr-Si alloy) / Nisil (Ni-Si alloy))High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B, R, S) types. They can withstand temperatures above 1200 °C. Sensitivity is about 39 µV/°C at 900°C, slightly lower than a Type K. Designed to be an improved type K, it is becoming more popular.

Silicon Diodes

• Temperature dependent IV curve

Refrigeration• Continous Flow• Cryogenic Bath (dewar)• Closed Cycle• Pulse tube• Thermo-electric• Adiabatic Demagnetization

Base Temperature, Cooling Power and Holding timeare the figures of merit.

(cryogen consumption, power, vibraton …)

Common Cryogens

• Liquid Nitrogen (LN2)– Boiling Point 77.36 K– Can be pumped down to ~ 64 K

• Liquid Helium– Boiling Point 4.2 K– Can be pumped down to ~ 1 K

Continous Flow Cryostats

Generally as a vacuum jacket

Cryogen is brought in by thermally insulated stainless steel tubing

A copper base is used as a cold mount for the sample

If LHe is used, multiple vacum jackets may be used to enhance base temperature

Continous Flow Cryostats

Different designs allow optical access or magneticFields to be applied

Bath (dewar) type cryostats

Advantages:

Long hold time

Low vibration (only boiling cryogen)

Below 4.2 K

Liquid Helium-4

Boiling Point 4.2 KCan be pumped down to 1 K

Helium 3 is an isotope of Helium

Boiling Point 3.2 KCan be pumped down to 0.3 K

LHe

pumped

Base temperatureWe can get to 1 K using only He4And 0.3 using He3.

Below 4.2 KSomething funny happens when youMix He3 and He4

DILUTION REFRIGERATIOR

Can get down to 10 mK regime

Complicated operationIn older systems

He3 is very expensive!

Now carbon cryopumps are used for pumping, resulting in complete sealed automatic systems

Closed Cycle Refrigerators• Similar to a household refrigerator• Uses compressed He gas to cool

No external Cryogen neededCan cool down to 10 K (4 K) in an hourA lot of noise !1.5 Watt @ 4.2 K available

Pulse Tube Refrigerators• Thermoacoustic device !• Only small number of moving part:

vibrating membrane

Characteristics: Applications:Refrigeration Capacity: 0.5 W @ 4.2 K Laboratory CryostatsOrientation: Vertical only Cryogenic Property MeasurementsPush-button Operation Optical Studies Magnetic StudiesLow-vibration

Thermoelectric coolers• Peltier Effect• Multiple stages can achieve 80 K• No vibration or cryogen• Compact• Long lifetime and low maintenance

Adiabatic demagnetization• Microkelvins in a box! “The adiabatic demagnetization

refrigerator (ADR) systems from Janis Research offer a simple method of achieving temperatures of 50 mK. These systems make use of two built in pills that operate at temperatures of 1 K and 50 mK, supported below a 4 K surface that is cooled with either liquid helium or a mechanical (pulse tube) cooler. These systems offer a typical hold time of two to three days below 100 mK, and require no active pumping on the cryostat. A recycling period of two to three hours is needed at the end of the three day period, to reduce the temperature back to 50 mK. “

Adiabatic demagnetizationMagnetocaloric effect

Works down to extreme low temperatures

(partly because it does not have a phase diagram like in a gaseous system)

GMCE: Giant Magnetocaloric effect