Low Temperature Methods (1K-300K)
By
Charulata Y. Barge,
Graduate student,
Prof. Zumbühl Group
Department of Physics
Date:- 3-11-2006
Outline
„
„
„
„
Cryogenics
Liquid Gases
Liquid Helium
Helium-4 refrigeration techniques
Cryogenics
„
„
„
„
„
„
„
„
„
„
„
„
„
„
Processes at low temperatures,
defined arbitrarily as below 150 K.
liquefaction and solidification of ambient gases
loss of ductility and embrittlement of some structural materials such as carbon steel
increase in the thermal conductivity to a maximum value
decrease in the heat capacity of solids
decrease in thermal noise and disorder of matter
appearance of quantum effects such as superconductivity and superfluidity
LT environments are maintained with cryogens (liquefied gases) or with cryogenic
refrigerators.
The temperature afforded by a cryogen ranges from its triple point to slightly below its
critical point
Commonly used cryogens are liquid helium-4 (down to 1 K), liquid hydrogen, and
liquid nitrogen
Less commonly used because of their expense are liquid helium-3 (down to 0.3 K) and
neon
Down to about 1.5 K, refrigeration cycles involve compression and expansion of
appropriately chosen gases
Latent heat of vaporization and the sensible heat of the gas (heat content of the gas) is
removed to liquefy a gas
Liquid Gases
„
„
„
„
„
„
„
„
Cyogenic liquids
Liquefied gases that are kept in their liquid state at very low
temperatures
Liquid gases are extremely cold
have boiling points below -150°C(- 238°F)
In gaseous state at normal temperatures and pressures.
Gases cooled below room temperature, an increase in pressure can
liquefy them
Different cryogens become liquids under different conditions of
temperature and pressure
All have two properties in common
•
•
they are extremely cold
small amounts of liquid can expand into very large volumes of gas.
Different types of cryogenic liquids
„
„
Each cryogenic liquid has its own specific properties
Inert Gases:
Do not react chemically
„ Do not burn or support combustion.
Examples:- nitrogen, helium, neon, argon and krypton
„
„
Flammable Gases:
can burn in air.
Examples:- hydrogen, methane and liquefied natural gas
„
„
Oxygen:
„
„
Materials considered as non-combustible can burn in the presence of liquid
oxygen.
Organic materials can react explosively with liquid oxygen.
How are cryogenic liquids contained?
„
„
„
used in thermally insulated containers
cryogenic liquid containers are specifically designed to withstand rapid
temperature changes and extreme differences in temperature
Liquid Dewar Flasks
„
„
„
Laboratory Liquid Dewar Flasks
„
„
„
non-pressurized, vacuum-jacketed vessels
low- boiling liquids have an outer vessel of liquid nitrogen for insulation.
wide-mouthed openings and do not have lids or covers.
primarily used in laboratories for temporary storage.
Liquid Cylinders
„
„
„
„
pressurized containers specifically designed for cryogenic liquids.
valves for filling and dispensing the cryogenic liquid,
pressure-control valve with a bursting disk as backup protection.
There are three major types of liquid cylinders which are designed for dispensing:
„
„
„
a) liquid or gas
b) only gas
c) only liquid
Liquid Nitrogen(N2)
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
inert, colorless, odorless, non-corrosive, nonflammable, and extremely cold.
Makes up the major portion of the atmosphere (78.03% by volume, 75.5% by weight).
Does not support combustion; however, it is not life supporting.
Inert except when heated to very high temperatures where it combines with some of
the more active metals,
Health Effects
nontoxic and inert,
Inhalation of nitrogen in excessive amounts can cause dizziness, nausea, vomiting, loss
of consciousness, and death.
Atomic Number:7
Atomic Mass Average:- 14.00674
Boiling Point: 77.5K -195.65°C -320.17°F
Density: 1.2506g/L @ 273K & 1atm
Heat of vaporization: 2.7928kJ/mol
Melting point: 63.29K -209.86°C -345.75°F
Molar Volume: 17.3 cm3/mole
Optical Refractive Index: 1.000298 (gas) 1.197 (liquid)
Physical State (at 20°C & 1atm): Gas
Specific Heat: 1.04J/gK
Handling and Storage
„
„
„
„
„
„
„
„
Store and use :- with adequate ventilation
Do not store in a confined space
Do not plug, remove, or tamper with any pressure relief device
Never allow any unprotected part of the body to come in contact with
uninsulated pipes or equipment that contains cryogenic product
The extremely cold metal will cause the flesh to stick fast and tear when one
attempts to withdraw from it. Use a suitable hand truck for container
movement
Containers should be handled and stored in an upright position
Use piping and equipment designed to withstand the pressures to be
encountered
On gas withdrawal systems, use a check valve or other protective apparatus in
any line or piping from the container to prevent reverse flow
Liquid Helium(He)
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
„
inert, colorless, odorless, non-corrosive, extremely cold, and nonflammable
non react with other elements or compounds under ordinary conditions
non-corrosive
Health Effects
Being odorless, colorless, tasteless, and nonirritating, helium has no warning properties
Humans possess no senses that can detect the presence of helium
a simple asphyxiant by displacing the oxygen in air to levels below that required to support
life
Inhalation of helium in excessive amounts can cause dizziness, nausea, vomiting, loss of
consciousness and death
Atomic Number: 2
Boiling point: 4.365KK
Density:- 0.1785g/L @ 273K & 1 atm
Enthalpy of Vaporization: 0.083 kJ/mole
Heat of vaporization: 0.0845kJ/mol
Melting point: 1.1K -272.05°C -458°F
Molar Volume: 31.8 cm3/mole
Optical Refractive Index: 1.000035 (gas) 1.028 (liquid)
Specific heat: 5.193J/gK
Handling and storage
„
„
„
„
„
„
„
„
„
Equipment:- liquid containers, vacuum jacketed transfer lines, process equipment, and
accessories needed to safely handle and use the product.
never tip, slide or roll liquid containers on their side.
Liquid helium container valves should never be left open to atmosphere for extended
periods.
Keep the fill/withdrawal vent outlets closed to prevent contamination.
Check the system regularly for frost accumulation.
Provide a safety relief valve on any part of the system where liquid can be trapped
between closed valves in lines or vessels.
Transfer liquid from containers by using the product vapor pressure or external gaseous
product pressure.
Provide protection for liquid helium containers against extremes of weather, where
outside storage areas are used.
carbon steel must be avoided in cryogenic service.
Liquid Helium
„
He is the only element that remains liquid at T=0.
„
The reasons
„
(a) large zero-point oscillations of light atoms, and
„
(b) the binding forces between the atoms are very weak.
Small atomic mass, large zero point energy
ƒ
The zero-point energy larger than the latent heat of evaporation
ƒ
the zero-point vibration amplitude is ~1/3 of the mean separation of atoms in the liquid state
ƒ
No static dipole moment:- closed electronic s shell
ƒ
for any processes that bring a system at T = 0 from one equilibrium state to another, ΔS = 0
„
Liquid Helium (He)
„
„
„
„
Two Isotopes
Helium 4 :- Common isotope
„ boils at a substantially lower temperature, 4.2 K
„ below 2.172 K the liquid exhibits the extraordinary properties of
superfluidity
Helium 3 :- rare isotope
„
normal boiling point of 3.2 K
„
superfluid transition at a very much lower temperature near 0.001 K
Both forms of helium remain in a liquid state at absolute zero.
Some Properties of Liquid Helium
Properties of Liquid Helium
Helium-4
Helium 3
Critical Temperature
5.2 K
3.3 K
Boiling Point at 1 atm
4.2 K
3.2 K
Minimum melting pressure
25 atm
29 atm at 0.3 K
Superfluid transition temperature
at saturated vapor pressure
2.17 K
1 mK in zero magnetic field
Type
Boson
Fermion
Outline
„
„
„
Cryogenics
Liquid Gases
Helium-4 refrigeration techniques
Helium-4
„
„
„
the most abundant of the two naturally-occurring isotopes of helium
A boson
The appearance in superfluid state is related to the Bose- Einstein
Condensation
Phase diagrame of Helium 4
The critical point
Tc = 5.20 K
Pc = 2.264 atm
Typical phase diagram
Helium I and Helium II states
„
Helium I :
„
„
„
„
Normal colourless liquid state
a very low viscosity and density
Normal fluid characteristics
Helium II:
„
„
„
„
„
high thermal conductivity
Absense of bubbling
heat input causes evaporation
Superfluid:- No measurable viscocity
liquid helium below the lambda point is viewed as containing a proportion
of helium atoms in a ground state, which are superfluid and flow with
exactly zero viscosity, and a proportion of helium atoms in an excited state,
which behave more like an ordinary fluid
Creeping Effect
„
„
„
„
When a surface extends past the level of
helium II, the helium II moves along the
surface, seemingly against the force of
gravity
will escape from a vessel that is not
sealed by creeping along the sides until it
reaches a warmer region where it
evaporates
moves in a 30 nm thick film Rollin film
regardless of surface material.
leaks rapidly through tiny openings
Helium 4 Cryostats
ƒ small latent heat i.e. Boils off easily
ƒ LHe cryostats has to be thermaly decoupled
ƒ Separating He vessel from outer world by a vacuum
ƒ container is made up from material with poor thermal conductivity
ƒ environment is shilded by surrounding LHe vessel with LN
ƒWraping the dewar in ‚super insulating‘ foil
ƒ Cryostats are equipped with superconducting magnet
ƒ Magnets are cooled below Tc by LHe
ƒ Nb alloys :- very large critical magnetic fields
Helium 4 bath cryostat
„
„
„
„
Sample is immersed in LHe
Sample cooled by pumping away He
vapor
LHe evaporates,cools liquid
Pumping speed and incoming heat
flux determines the lowest possible
temperature
Helium 4 Gas flow cryostat
ƒThe sample sits in flow of cold
helium gas
ƒLHe enters via needle valve
ƒadditional vacuum chamber to
thermaly decouple Sample chamber
ƒContunous variation of
temperature between 1.2K and room
temperature
Working mechanism
„
Claussius Clapeyron equation
⎛ dP⎞ Sgas−Sliq L
=
⎜ ⎟ =
⎝dT⎠vap Vgas−Vliq TΔV
For the liquid-gas phase transition, we can make the following
reasonable assumptions:
„
the molar volume of the liquid is much smaller than the molar
volume of the gas
„
the molar volume of gas is given by the ideal gas law V = RT/P
„
the latent heat is almost T-independent
L
LP
⎛ dP ⎞
=
⎜
⎟ ≈
2
⎝ dT ⎠ vap TVgas RT
dP L dT
=
P R T2
⎛ L ⎞
Pvap ∝ exp⎜ −
⎟
RT
⎝
⎠
Working mechanism
„
Cooling power (P):- latent heat taken from from liquid per evaporated atom x
no. of atoms evaporated per time
„
„
P= (dn/dt)*L
dn/dt determined by pumping speed (dV/dt)
dn/dt =1/mHe (dM/dt)
= 1/mHe *ρ (dV/dt)
=P(T)/kBT (dV/dt)
Cooling power drops exponentially as T decreases
Steady state:- P=heat load of LHe
With 10m3/h, T reached 1.2K
For lower temperature, powerful pumps or Helium 3
Thank you for your attention