15-03-2011, 08:49 PM
Magnetic Refrigeration
Feby Philip Abraham , Ananthu Sivan
S4 Departement of Mechanical Engineering
Mohandas College Of Engineering and Technology
[attachment=10245]
Abstract
A cooling system consists of a device or devices used to lower the temperature of a defined region in space through
some cooling process. Currently, the most popular commercial cooling agent is the refrigerant. A refrigerant in its
general sense is what makes refrigerator cool foods, and it also makes air conditioners and other appliances
perform their respective duties. A typical consumer based refrigerator lowers temperatures by modulating a gas
compression-expansion cycle, to cool a refrigerant fluid which has been warmed by the contents of the refrigerator
(i.e. the food inside). Typical refrigerants used in refrigerators include ammonia, methyl chloride, and sulfur
dioxide, all of which are toxic. To mitigate the risks associated with toxic refrigerants, a collaboration by
Frigidaire, General Motors, and DuPont netted the development of Freon (or R12), a chlorofluorocarbon. Freon is
a non-flammable and non-toxic, but ozone- depleting gas. Because of the damaging effects of Freon to the ozone
layer, there has been much interest in targeting other refrigerants. The popular refrigerant R134a (called Suva by
DuPont) is currently used in most refrigerators, but American and international laws are beginning to phase out
this refrigerant as well. The future seems ripe for new refrigeration technology. This has led the world to look for a
better source of refrigeration, and magnetic refrigeration is certainly one of the best options if we consider the
environmental aspects. There are two attractive reasons why magnetic refrigeration research continues. While a
magnetic refrigerator would cost more than today's refrigerator at purchase, it could conserve over and above 20%
more energy than current expansion-compression refrigerators, drastically reducing operating costs. The other
attraction to magnetic refrigeration is the ecological impact a magnetic refrigerator would bring should it supplant
current technologies. Not only would ozone-depleting refrigerant concerns be calmed, but the energy savings itself
would lessen the strain our household appliances put on our environment.
Introduction
Magnetic refrigeration is a cooling technology based
on the magneto caloric effect. This technique can be
used to attain extremely low temperatures (well
below 1 Kelvin), as well as the ranges used in
common refrigerators, depending on the design of the
system. The fundamental principle was suggested by
Debye (1926) and Giauque (1927) and the first
working magnetic refrigerators were constructed by
several groups beginning in 1933. Magnetic
refrigeration was the first method developed for
cooling below about 0.3 Kelvin (a temperature
attainable by3He/4He dilution refrigeration).
The Magnetocaloric Effect
The Magnetocaloric effect (MCE, from magnet and
calorie) is a magneto- thermodynamic phenomenon
in which a reversible change in temperature of a
suitable material is caused by exposing the material
to a changing magnetic field. This is also known as
adiabatic demagnetization by low temperature
physicists, due to the application of the process
specifically to effect a temperature drop. In that part of the overall refrigeration process, a decrease in the
strength of an externally applied magnetic field
allows the magnetic domains of a chosen
(magnetocaloric) material to become disoriented
from the magnetic field by the agitating action of the
thermal energy (phonons) present in the material. If
the material is isolated so that no energy is allowed to
(e) migrate into the material during this time (i.e. an
adiabatic process), the temperature drops as the
domains absorb the thermal energy to perform their
reorientation. The randomization of the domains
occurs in a similar fashion to the randomization at the
Curie temperature, except that magnetic dipoles
overcome a decreasing external magnetic field while
energy remains constant, instead of magnetic
domains being disrupted from internal
ferromagnetism as energy is added. One of the most
notable examples of the magnetocaloric effect is in
the chemical element gadolinium and some of its
alloys. Gadolinium's temperature is observed to
increase when it enters certain magnetic fields. When
it leaves the magnetic field, the temperature returns to
normal. The effect is considerably stronger for the
gadolinium alloy Gd5 (Si2Ge2). Praseodymium
alloyed with nickel (PrNi5) has such a strong
magnetocaloric effect that it has allowed scientists to
approach within one thousandth of a degree of
Components required for construction
Magnets:Magnets provide the magnetic field to the
material so that they can loose or gain the heat to the
surrounding and from the space to be cooled
respectively
Hot Heat exchanger:The hot heat exchanger absorbs
the heat from the material used and gives off to the
surrounding. It makes the transfer of heat much
effective
Cold Heat Exchanger:The cold heat exchanger
absorbs the heat from the space to be cooled and
Magnetic Refrigeration Cycle
The cycle is performed as a refrigeration cycle,
analogous to the Carnot cycle, and can be described
at a starting point whereby the chosen working
substance is introduced into a magnetic field (i.e. the
magnetic flux density is increased). The working
material is the refrigerant, and starts in thermal
equilibrium with the refrigerated environment
•Adiabatic magnetization: The substance is placed
in an insulated environment. The increasing external
magnetic field (+H) causes the magnetic dipoles of
the atoms to align, thereby decreasing the
material'smagneticentrop y and heat capacity. Since
overall energy is not lost (yet) and therefore total
entropy is not reduced (according to thermodynamic
laws), the net result is that the item heats up (T +
ΔTad).
•Isomagnetic enthalpic transfer: This added heat
can then be removed by a fluid like water or helium
for example (-Q). The magnetic field is held constant
to prevent the dipoles from reabsorbing the heat.
Once sufficiently cooled, the magnetocaloric material
and the coolant are separated (H=0).
•Adiabatic demagnetization: The substance is
returned to another adiabatic (insulated) condition so
the total entropy remains constant. However, this
time the magnetic field is decreased, the thermal
energy causes the domains to overcome the field, and
thus the sample cools (i.e. an adiabatic temperature
change). Energy (and entropy) transfers from thermal
entropy to magnetic entropy (disorder of the
magnetic dipoles).
•Isomagnetic entropic transfer: The magnetic field
is held constant to prevent the material from heating
back up. The material is placed in thermal contact
with the environment being refrigerated. Because the
working material is cooler than the refrigerated
environment (by design), heat energy migrates into
the working material (+Q).
Once the refrigerant and refrigerated environment is
in thermal equilibrium, the cycle begins anew
Applied Technique
The basic operating principle of an ADR is the use of
a strong magnetic field to control the entropy of a
sample of material, often called the "refrigerant".
Magnetic field constrains the orientation of magnetic
dipoles in the refrigerant. The stronger the magnetic
field, the more aligned the dipoles are, and this
corresponds to lower entropy and heat capacity
because the material has (effectively) lost some of its
internal degrees of freedom. If the refrigerant is kept
at a constant temperature through thermal contact
with a heat sink (usually liquid helium) while the
magnetic field is switched on, the refrigerant must
lose some energy because it is equilibrated with the
heat sink. When the magnetic field is subsequently
switched off, the heat capacity of the refrigerant rises
again because the degrees of freedom associated with
orientation of the dipoles are once again liberated,
pulling their share of equipartitioned energy from the
motion of the molecules, thereby lowering the overall
temperature of a system with decreased energy. Since
the system is now insulated when the magnetic field
is switched off, the process is adiabatic, i.e. the
system can no longer exchange energy with its
surroundings (the heat sink), and its temperature
decreases below its initial value, that of the heat sink.
The operation of a standard ADR proceeds roughly
as follows. First, a strong magnetic field is applied to
the refrigerant, forcing its various magnetic dipoles to
align and putting these degrees of freedom of the
refrigerant into a state of lowered entropy. The heat
sink then absorbs the heat released by the refrigerant
due to its loss of entropy. Thermal contact with the
heat sink is then broken so that the system is
insulated, and the magnetic field is switched off,
increasing the heat capacity of the refrigerant, thus
decreasing its temperature below the temperature of
the He heat sink. In practice, the magnetic field is
decreased slowly in order to provide continuous
cooling and keep the sample at an approximately
constant low temperature. Once the field falls to zero
(or to some low limiting value determined by the
properties of the refrigerant), the cooling power of
the ADR vanishes, and heat leaks will cause the
refrigerant to warm up.
Working Materials
The magnetocaloric effect is an intrinsic property of a
magnetic solid. This thermal response of a solid to
the application or removal of magnetic fields is
maximized when the solid is near its magnetic
ordering temperature. The magnitudes of the
magnetic entropy and the adiabatic temperature
changes are strongly dependent upon the magnetic
order process: the magnitude is generally small in
antiferromagnets, ferrimagnets and spin glass
systems; it can be substantial for normal
ferromagnets which undergo a second order magnetic
transition; and it is generally the largest for a
ferromagnet which undergoes a first order magnetic
transition.Also, crystalline electric fields and pressure
can have a substantial influence on magnetic entropy
and adiabatic temperature changes. Currently, alloys
of gadolinium producing 3 to 4 K per tesla of change
in a magnetic field can be used for magnetic
refrigeration or power generation purposes.Recent
research on materials that exhibit a giant entropy
change showed that Gd5(SixGe1 − x)4, La(FexSi1 −
x)13Hx and MnFeP1 − xAsx alloys, for example, are
some of the most promising substitutes for
Gadolinium and its alloys (GdDy, GdTy, etc...).
These materials are called giant magnetocaloric
effect materials (GMCE). Gadolinium and its alloys
are the best material available today for magnetic
refrigeration near room temperature since they
undergo second-order phase transitions which have
no magnetic or thermal hysteresis involved
Paramagnetic Salts
The originally suggested refrigerant was a
paramagnetic salt, such as cerium magnesium nitrate.
The active magnetic dipoles in this case are those of
the electron shells of the paramagnetic atoms. In a
paramagnetic salt ADR, the heat sink is usually
provided by a pumped4He (about 1.2 K) or3He
(about 0.3 K) cryostat. An easily attainable 1 tesla
magnetic field is generally required for the initial
magnetization. The minimum temperature attainable
is determined by the self-magnetization tendencies of
the chosen refrigerant salt, but temperatures from 1 to
100 mK are accessible. Dilution refrigerators had for
many years supplanted paramagnetic saltADRs, but
interest in space-based and simple to use lab-ADRs
has recently revived the field. Eventually
paramagnetic salts become either diamagnetic or
ferromagnetic, limiting the lowest temperature which
can be reached using this method.
Nuclear Demagnetisation
One variant of adiabatic demagnetization that
continues to find substantial research application is
nuclear demagnetization refrigeration (NDR). NDR
follows the same principle described above, but in
this case the cooling power arises from the magnetic
dipoles of the nuclei of the refrigerant atoms, rather
than their electron configurations. Since these dipoles are of much smaller magnitude, they are less prone to
self-alignment and have lower intrinsic minimum
fields. This allows NDR to cool the nuclear spin
system to very low temperatures, often 1 µK or
below. Unfortunately, the small magnitudes of
nuclear magnetic dipoles also make them less
inclined to align to external fields. Magnetic fields of
3 teslas or greater are often needed for the initial
magnetization step of NDR.In NDR systems, the
initial heat sink must sit at very low temperatures
(10–100 mK). This precooling is often provided by
the mixing chamber of a dilution refrigerator or a
paramagnetic salt ADR stage.
Feby Philip Abraham , Ananthu Sivan
S4 Departement of Mechanical Engineering
Mohandas College Of Engineering and Technology
[attachment=10245]
Abstract
A cooling system consists of a device or devices used to lower the temperature of a defined region in space through
some cooling process. Currently, the most popular commercial cooling agent is the refrigerant. A refrigerant in its
general sense is what makes refrigerator cool foods, and it also makes air conditioners and other appliances
perform their respective duties. A typical consumer based refrigerator lowers temperatures by modulating a gas
compression-expansion cycle, to cool a refrigerant fluid which has been warmed by the contents of the refrigerator
(i.e. the food inside). Typical refrigerants used in refrigerators include ammonia, methyl chloride, and sulfur
dioxide, all of which are toxic. To mitigate the risks associated with toxic refrigerants, a collaboration by
Frigidaire, General Motors, and DuPont netted the development of Freon (or R12), a chlorofluorocarbon. Freon is
a non-flammable and non-toxic, but ozone- depleting gas. Because of the damaging effects of Freon to the ozone
layer, there has been much interest in targeting other refrigerants. The popular refrigerant R134a (called Suva by
DuPont) is currently used in most refrigerators, but American and international laws are beginning to phase out
this refrigerant as well. The future seems ripe for new refrigeration technology. This has led the world to look for a
better source of refrigeration, and magnetic refrigeration is certainly one of the best options if we consider the
environmental aspects. There are two attractive reasons why magnetic refrigeration research continues. While a
magnetic refrigerator would cost more than today's refrigerator at purchase, it could conserve over and above 20%
more energy than current expansion-compression refrigerators, drastically reducing operating costs. The other
attraction to magnetic refrigeration is the ecological impact a magnetic refrigerator would bring should it supplant
current technologies. Not only would ozone-depleting refrigerant concerns be calmed, but the energy savings itself
would lessen the strain our household appliances put on our environment.
Introduction
Magnetic refrigeration is a cooling technology based
on the magneto caloric effect. This technique can be
used to attain extremely low temperatures (well
below 1 Kelvin), as well as the ranges used in
common refrigerators, depending on the design of the
system. The fundamental principle was suggested by
Debye (1926) and Giauque (1927) and the first
working magnetic refrigerators were constructed by
several groups beginning in 1933. Magnetic
refrigeration was the first method developed for
cooling below about 0.3 Kelvin (a temperature
attainable by3He/4He dilution refrigeration).
The Magnetocaloric Effect
The Magnetocaloric effect (MCE, from magnet and
calorie) is a magneto- thermodynamic phenomenon
in which a reversible change in temperature of a
suitable material is caused by exposing the material
to a changing magnetic field. This is also known as
adiabatic demagnetization by low temperature
physicists, due to the application of the process
specifically to effect a temperature drop. In that part of the overall refrigeration process, a decrease in the
strength of an externally applied magnetic field
allows the magnetic domains of a chosen
(magnetocaloric) material to become disoriented
from the magnetic field by the agitating action of the
thermal energy (phonons) present in the material. If
the material is isolated so that no energy is allowed to
(e) migrate into the material during this time (i.e. an
adiabatic process), the temperature drops as the
domains absorb the thermal energy to perform their
reorientation. The randomization of the domains
occurs in a similar fashion to the randomization at the
Curie temperature, except that magnetic dipoles
overcome a decreasing external magnetic field while
energy remains constant, instead of magnetic
domains being disrupted from internal
ferromagnetism as energy is added. One of the most
notable examples of the magnetocaloric effect is in
the chemical element gadolinium and some of its
alloys. Gadolinium's temperature is observed to
increase when it enters certain magnetic fields. When
it leaves the magnetic field, the temperature returns to
normal. The effect is considerably stronger for the
gadolinium alloy Gd5 (Si2Ge2). Praseodymium
alloyed with nickel (PrNi5) has such a strong
magnetocaloric effect that it has allowed scientists to
approach within one thousandth of a degree of
Components required for construction
Magnets:Magnets provide the magnetic field to the
material so that they can loose or gain the heat to the
surrounding and from the space to be cooled
respectively
Hot Heat exchanger:The hot heat exchanger absorbs
the heat from the material used and gives off to the
surrounding. It makes the transfer of heat much
effective
Cold Heat Exchanger:The cold heat exchanger
absorbs the heat from the space to be cooled and
Magnetic Refrigeration Cycle
The cycle is performed as a refrigeration cycle,
analogous to the Carnot cycle, and can be described
at a starting point whereby the chosen working
substance is introduced into a magnetic field (i.e. the
magnetic flux density is increased). The working
material is the refrigerant, and starts in thermal
equilibrium with the refrigerated environment
•Adiabatic magnetization: The substance is placed
in an insulated environment. The increasing external
magnetic field (+H) causes the magnetic dipoles of
the atoms to align, thereby decreasing the
material'smagneticentrop y and heat capacity. Since
overall energy is not lost (yet) and therefore total
entropy is not reduced (according to thermodynamic
laws), the net result is that the item heats up (T +
ΔTad).
•Isomagnetic enthalpic transfer: This added heat
can then be removed by a fluid like water or helium
for example (-Q). The magnetic field is held constant
to prevent the dipoles from reabsorbing the heat.
Once sufficiently cooled, the magnetocaloric material
and the coolant are separated (H=0).
•Adiabatic demagnetization: The substance is
returned to another adiabatic (insulated) condition so
the total entropy remains constant. However, this
time the magnetic field is decreased, the thermal
energy causes the domains to overcome the field, and
thus the sample cools (i.e. an adiabatic temperature
change). Energy (and entropy) transfers from thermal
entropy to magnetic entropy (disorder of the
magnetic dipoles).
•Isomagnetic entropic transfer: The magnetic field
is held constant to prevent the material from heating
back up. The material is placed in thermal contact
with the environment being refrigerated. Because the
working material is cooler than the refrigerated
environment (by design), heat energy migrates into
the working material (+Q).
Once the refrigerant and refrigerated environment is
in thermal equilibrium, the cycle begins anew
Applied Technique
The basic operating principle of an ADR is the use of
a strong magnetic field to control the entropy of a
sample of material, often called the "refrigerant".
Magnetic field constrains the orientation of magnetic
dipoles in the refrigerant. The stronger the magnetic
field, the more aligned the dipoles are, and this
corresponds to lower entropy and heat capacity
because the material has (effectively) lost some of its
internal degrees of freedom. If the refrigerant is kept
at a constant temperature through thermal contact
with a heat sink (usually liquid helium) while the
magnetic field is switched on, the refrigerant must
lose some energy because it is equilibrated with the
heat sink. When the magnetic field is subsequently
switched off, the heat capacity of the refrigerant rises
again because the degrees of freedom associated with
orientation of the dipoles are once again liberated,
pulling their share of equipartitioned energy from the
motion of the molecules, thereby lowering the overall
temperature of a system with decreased energy. Since
the system is now insulated when the magnetic field
is switched off, the process is adiabatic, i.e. the
system can no longer exchange energy with its
surroundings (the heat sink), and its temperature
decreases below its initial value, that of the heat sink.
The operation of a standard ADR proceeds roughly
as follows. First, a strong magnetic field is applied to
the refrigerant, forcing its various magnetic dipoles to
align and putting these degrees of freedom of the
refrigerant into a state of lowered entropy. The heat
sink then absorbs the heat released by the refrigerant
due to its loss of entropy. Thermal contact with the
heat sink is then broken so that the system is
insulated, and the magnetic field is switched off,
increasing the heat capacity of the refrigerant, thus
decreasing its temperature below the temperature of
the He heat sink. In practice, the magnetic field is
decreased slowly in order to provide continuous
cooling and keep the sample at an approximately
constant low temperature. Once the field falls to zero
(or to some low limiting value determined by the
properties of the refrigerant), the cooling power of
the ADR vanishes, and heat leaks will cause the
refrigerant to warm up.
Working Materials
The magnetocaloric effect is an intrinsic property of a
magnetic solid. This thermal response of a solid to
the application or removal of magnetic fields is
maximized when the solid is near its magnetic
ordering temperature. The magnitudes of the
magnetic entropy and the adiabatic temperature
changes are strongly dependent upon the magnetic
order process: the magnitude is generally small in
antiferromagnets, ferrimagnets and spin glass
systems; it can be substantial for normal
ferromagnets which undergo a second order magnetic
transition; and it is generally the largest for a
ferromagnet which undergoes a first order magnetic
transition.Also, crystalline electric fields and pressure
can have a substantial influence on magnetic entropy
and adiabatic temperature changes. Currently, alloys
of gadolinium producing 3 to 4 K per tesla of change
in a magnetic field can be used for magnetic
refrigeration or power generation purposes.Recent
research on materials that exhibit a giant entropy
change showed that Gd5(SixGe1 − x)4, La(FexSi1 −
x)13Hx and MnFeP1 − xAsx alloys, for example, are
some of the most promising substitutes for
Gadolinium and its alloys (GdDy, GdTy, etc...).
These materials are called giant magnetocaloric
effect materials (GMCE). Gadolinium and its alloys
are the best material available today for magnetic
refrigeration near room temperature since they
undergo second-order phase transitions which have
no magnetic or thermal hysteresis involved
Paramagnetic Salts
The originally suggested refrigerant was a
paramagnetic salt, such as cerium magnesium nitrate.
The active magnetic dipoles in this case are those of
the electron shells of the paramagnetic atoms. In a
paramagnetic salt ADR, the heat sink is usually
provided by a pumped4He (about 1.2 K) or3He
(about 0.3 K) cryostat. An easily attainable 1 tesla
magnetic field is generally required for the initial
magnetization. The minimum temperature attainable
is determined by the self-magnetization tendencies of
the chosen refrigerant salt, but temperatures from 1 to
100 mK are accessible. Dilution refrigerators had for
many years supplanted paramagnetic saltADRs, but
interest in space-based and simple to use lab-ADRs
has recently revived the field. Eventually
paramagnetic salts become either diamagnetic or
ferromagnetic, limiting the lowest temperature which
can be reached using this method.
Nuclear Demagnetisation
One variant of adiabatic demagnetization that
continues to find substantial research application is
nuclear demagnetization refrigeration (NDR). NDR
follows the same principle described above, but in
this case the cooling power arises from the magnetic
dipoles of the nuclei of the refrigerant atoms, rather
than their electron configurations. Since these dipoles are of much smaller magnitude, they are less prone to
self-alignment and have lower intrinsic minimum
fields. This allows NDR to cool the nuclear spin
system to very low temperatures, often 1 µK or
below. Unfortunately, the small magnitudes of
nuclear magnetic dipoles also make them less
inclined to align to external fields. Magnetic fields of
3 teslas or greater are often needed for the initial
magnetization step of NDR.In NDR systems, the
initial heat sink must sit at very low temperatures
(10–100 mK). This precooling is often provided by
the mixing chamber of a dilution refrigerator or a
paramagnetic salt ADR stage.
