CRYOGENICS. MSN 506/Phys 580

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1 CRYOGENICS MSN 506/Phys 580

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

3 Why do we need low temperatures? Physics Reduced thermal energy k B T 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 Biology Sample preservation Certain techniques for studying otherwise elastic samples

4 Basic refrigeration Used in daily life Household refrigeration Industrial refrigeration Reverse heat engine

5 Heat Capacity Amount of heat given to the material to raise its temperature under sepcified conditions Specific heat capacity Q = m c ΔT Joule/g/K Dependent on temperature for solids Dependent on pressure and temperature for gases

6 Heat conduction Q T H T L R capacitor V H I V L Equivalent circuit

7 Heat conduction Multiple Physical Mechanisms Conduction Convection Radiation Conduction Solids Convection Fluids (gases and liquids) Radiation All materials

8 Thermal conductivity thermal conductivity = heat flow rate distance / (area temperature difference) Unit : W/m/K Glass 1.1 Aluminium 220 Stainless steel 15 Diamond 2000 Silver 400 Copper 385 Air (STP) Temperature dependent

9 Thermal conductivity 15 mm plate separation at 200 K Vacuum is a good insulator Low molecular mass gases are good conductors of heat

10 Thermal resistance due to contact

11 Thermal circuits Cooling Power Heat load due to imperfect insulation or vacuum, And conduction by Radiation sample Heat Load from experiment (electronics, lasers etc.) Mechanical support acts as a thermal resistor

12 Radiation Stefan-Boltzman Law at 100 K the energy flux density is 5.67 W/m 2, at 1000 K 56,700 W/m 2 May become important only at extreme low temperatures Or may affect the hold time of cryogenic storage dewars

13 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.

14 Monitoring Temperature Thermistors Thermocouples Diodes

15 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.

16 Metals Monitoring Temperature

17 Monitoring Temperature Non-Metals

18 Monitoring Temperature

19 Monitoring Temperature

20 Monitoring Temperature

21 Monitoring Temperature Thermocouples work better at high temperatures (Ovens) Two wires of different materials made into junctions

22 Monitoring Temperature A 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 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.

23 Silicon Diodes Temperature dependent IV curve

24 Refrigeration Continous Flow Cryogenic Bath (dewar) Closed Cycle Pulse tube Thermo-electric Adiabatic Demagnetization Base Temperature, Cooling Power and Holding time are the figures of merit. (cryogen consumption, power, vibraton )

25 Common Cryogens Liquid Nitrogen (LN 2 ) Boiling Point K Can be pumped down to ~ 64 K Liquid Helium Boiling Point 4.2 K Can be pumped down to ~ 1 K

26 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

27 Continous Flow Cryostats Different designs allow optical access or magnetic Fields to be applied

28 Bath (dewar) type cryostats Advantages: Long hold time Low vibration (only boiling cryogen)

29 Below 4.2 K pumped Liquid Helium-4 Boiling Point 4.2 K Can be pumped down to 1 K LHe Helium 3 is an isotope of Helium Boiling Point 3.2 K Can be pumped down to 0.3 K We can get to 1 K using only He4 And 0.3 using He3. Base temperature

30 Below 4.2 K Something funny happens when you Mix He3 and He4 DILUTION REFRIGERATIOR Can get down to 10 mk regime Complicated operation In older systems He3 is very expensive! Now carbon cryopumps are used for pumping, resulting in complete sealed automatic systems

31 Closed Cycle Refrigerators Similar to a household refrigerator Uses compressed He gas to cool K available No external Cryogen needed Can cool down to 10 K (4 K) in an hour A lot of noise!

32 Pulse Tube Refrigerators Thermoacoustic device! Only small number of moving part: vibrating membrane Characteristics: Applications: Refrigeration Capacity: K Laboratory Cryostats Orientation: Vertical only Cryogenic Property Measurements Push-button Operation Optical Studies Magnetic Studies Low-vibration

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

34 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.

35 Adiabatic demagnetization Magnetocaloric 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