Page 1Adiabatic demagnetization INTRODUCTION Dictionary P P h hot t o on nC Co ool l i in n g g b b y y A Ad di i a a b ba at t i ic D De e m ma a g g n ne et t i iza a t t i i o on n means the process of cooling photons in an atom by the method called a diabatic demagnetization. According to third law of thermodynamics cooling of a material is possible only till absolute o. i.e. 0 Kelvin. This temperature is reached by freezing the movement of molecules of a substance. Encyclopedia Adiabatic Demagnetization Refrigerators (ADRs) have been used as reliable tools for cooling to temperatures below 100 mK for over 60 years [1]. In the 1970s, however, ADRs took a back seat in the t ypical low temperature laboratory environment to the dilution refrigerator, which offers higher cooling power (although at the expense of additional complication) . The last several years have witnessed the development of two-stage ADRs which, when combined with a low-temperature mechanical cryocooler, will achieve temperatures below 100 mKwithout cryog ens. In an ADR, a paramagnetic material is suspended in a magne tic field, typically provided by a s upercond ucting magnet. The refrigerant is t hermally isolated from higher temperature stages of the cryostat and thermally connected to the parts that are to be cooled. Ma ny suitable paramagnetic materials are salts of hydration, and the salt is housed in a hermetically sealed container (the ³salt pill´) to prevent dehydrati on in the cryosta t¶s vacuum. The temperature of a substance is determined by the average velocity of its molecules: the faster they move, the warmer the substance. At absolute zero molecules have minimal kinetic energy (or zero-point energy) and heat energy cannot be extracted from them. The molecules are not motionless, however, due to the uncertainty principle of quantum mechanics, which entails that the atoms cannot have both a fixed position and zero momentum at the sa me time; instead, the molecules of a substance at absolute zero are always "wiggling" in some manner. Absolute zero is zero Kelvin, equal to -273.15 degrees Celsius and -459.67 degrees
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PPhhoottoonn CCoooolliinngg b byy AAddiiaa b baattiicc DDeemmaaggnneettiizzaattiioonn means the process of cooling photons in an
atom by the method called adiabatic demagnetization.
According to third law of thermodynamics cooling of a material is possible only till absolute
o. i.e. 0 Kelvin. This temperature is reached by freezing the movement of molecules of a
substance.
Encyclopedia
Adiabatic Demagnetization Refrigerators (ADRs) have been used as reliable tools for cooling
to temperatures below 100 mK for over 60 years [1]. In the 1970s, however, ADRs took a
back seat in the typical low temperature laboratory environment to the dilution refrigerator,
which offers higher cooling power (although at the expense of additional complication). The
last several years have witnessed the development of two-stage ADRs which, when combined
with a low-temperature mechanical cryocooler, will achieve temperatures below 100 mK
without cryogens. In an ADR, a paramagnetic material is suspended in a magnetic field,
typically provided by a superconducting magnet. The refrigerant is thermally isolated from
higher temperature stages of the cryostat and thermally connected
to the parts that are to be cooled. Many suitable paramagnetic materials are salts of hydration,
and the salt is housed in a hermetically sealed container (the ³salt pill´) to prevent
dehydration in the cryostat¶s vacuum.
The temperature of a substance is determined by the average velocity of its molecules: the
faster they move, the warmer the substance. At absolute zero molecules have minimal kineticenergy (or zero-point energy) and heat energy cannot be extracted from them. The molecules
are not motionless, however, due to the uncertainty principle of quantum mechanics, which
entails that the atoms cannot have both a fixed position and zero momentum at the same time;
instead, the molecules of a substance at absolute zero are always "wiggling" in some manner.
Absolute zero is zero Kelvin, equal to -273.15 degrees Celsius and -459.67 degrees
Here the symbol " " is a representation of a finite increment, so that S indicates a "change"
or "increment" in S, as in S = S1 - S2, where S1 and S2 are the entropies of two different
equilibrium states, and likewise Q. If Q is positive, then so is S, so if the internal heat
energy goes up, while the temperature remains fixed, then the entropy S goes up. And, if the
internal heat energy Q goes down ( Q is a negative number), then the entropy will go down
too.
Clausius and the others, especially Carnot, were much interested in the ability to convert
mechanical work into heat energy, and vice versa. This idea can lead us to an alternate form
for equation 2, that will be useful later on. Suppose you pump energy, U, into a system,
Part of the energy goes into the internal heat content, Q, making Q a positive quantity, but
not all of it. Some of that energy could easily be expressed as an amount of mechanical work
done by the system ( W, such as a hot gas pushing against a piston in a car engine). So thatQ = U - W, where U is the energy input to the system, and W is the part of that
energy that goes into doing work. The difference between them is the amount of energy that
does not participate in the work, and goes into the heat reservoir as Q. So a simple
substitution allows equation 2 to be re-written as equation 3.
S = ( U - W)/T---------------------------------------------------------------------------------------3
This alternate form of the equation works for heat taken out of a system ( U is negative) or
work done on a system ( W is negative), just as well. So it gives the better idea of the
classical relation between work, energy and entropy.
THIRD LAW OF THERMODYNAMICS
Third law states that ³It is im£ ossibl ¤ f or any materi al b y any process t o cool below absolute
zero´
This postulate related to but independent of the second law is that it is impossible to cool a
body to absolute zero by any finite process. Although one can approach absolute zero as
closely as one desires, one cannot actually reach this limit. The third law of thermodynamics,
formulated by Walter Nernst and also known as the Nernst heat theorem, states that if one
could reach absolute zero, all bodies would have the same entropy. In other words, a body at
absolute zero could exist in only one possible state, which would possess a definite energy,
called the zero-point energy. This state is defined as having zero entropy.
so to cool any material to zero Kelvin or near zero Kelvin we require to stop the motion of
the molecules of an atom. As this is not possible it is impossible to cool any material to zeroKelvin. Still scientists have reached temperature as low as 50 micro Kelvin, Which is almost
considered as zero Kelvin
GENERAL COOLING METHODS
� Freezing to 00 c (vapor compression or absorption)
� Dilution refrigeration
� Adiabatic demagnetization refrigeration.
VAPOR COMPRESSION REFRIGERATION
A simple vapor compression refrigeration system consists of the following equipments¶)
Compressor ii) Condenser iii) Expansion valve iv) Evaporator The schematic diagram of the
arrangement is as shown in Fig.6.5. The low temperature, low pressure vapor at state B is
compressed by a compressor to high temperature and pressure vapor at state C. This vapor is
condensed into high pressure vapor at state D in the condenser and then passes through the
expansion valve. Here, the vapor is throttled down to a low pressure liquid and passed on to
an evaporator, where it absorbs heat from the surroundings from the circulating fluid (being
refrigerated) and vaporizes into low pressure vapor at state B. The cycle then repeats.The