04-07-2013, 03:57 PM
MAGNETIC REFRIGERATION
MAGNETIC REFRIGERATION.doc (Size: 89.5 KB / Downloads: 22)
ABSTRACT:
Refrigeration is a phenomenon in which the heat transfer from it to the external environment ,cooling the content to a temperature below the ambient temperature.
In recent times many inventions are made and one of the new generation refrigeration is magnetic refrigeration.
INTRODUCTION:
Magnetic refrigeration is a cooling technology based on the magnetocaloric 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 kelvins
The magnetocaloric effect:
The Magnetocaloric effect 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 material to become disoriented from the magnetic field by the agitating action of the thermal energy present in the material. If the material is isolated so that no energy is allowed to migrate into the material during this time 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.
Example of magnetocaloric effect:
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 absolute zero.
Thermodynamic 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.
Applied technique:
1. The basic operating principle of an Adiabatic Demagnetization Refrigerator (ADR) is the use of a strong magnetic field to control the entropy of a sample of material, often called the "refrigerant".
2. 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 lost some of its internal degrees of freedom. If the refrigerant is kept at a constant temperature through thermal contact with a heat sink while the magnetic field is switched on, the refrigerant must lose some energy because it is equilibrated with the heat sink.
3. 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.
4. 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, and its temperature decreases below its initial value, that of the heat sink.
General procedure:
1. 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.
2. The heat sink then absorbs the heat released by the refrigerant due to its loss of entropy.
3. 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.
MAGNETIC REFRIGERATION.doc (Size: 89.5 KB / Downloads: 22)
ABSTRACT:
Refrigeration is a phenomenon in which the heat transfer from it to the external environment ,cooling the content to a temperature below the ambient temperature.
In recent times many inventions are made and one of the new generation refrigeration is magnetic refrigeration.
INTRODUCTION:
Magnetic refrigeration is a cooling technology based on the magnetocaloric 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 kelvins
The magnetocaloric effect:
The Magnetocaloric effect 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 material to become disoriented from the magnetic field by the agitating action of the thermal energy present in the material. If the material is isolated so that no energy is allowed to migrate into the material during this time 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.
Example of magnetocaloric effect:
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 absolute zero.
Thermodynamic 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.
Applied technique:
1. The basic operating principle of an Adiabatic Demagnetization Refrigerator (ADR) is the use of a strong magnetic field to control the entropy of a sample of material, often called the "refrigerant".
2. 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 lost some of its internal degrees of freedom. If the refrigerant is kept at a constant temperature through thermal contact with a heat sink while the magnetic field is switched on, the refrigerant must lose some energy because it is equilibrated with the heat sink.
3. 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.
4. 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, and its temperature decreases below its initial value, that of the heat sink.
General procedure:
1. 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.
2. The heat sink then absorbs the heat released by the refrigerant due to its loss of entropy.
3. 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.