Microstructures of Plain Carbon Steel

                                                              0.1% carbon steel

 
                                                         0.2% carbon steel

   
                                                          0.4% carbon steel

                                                          0.6% carbon steel

                                                              0.75% carbon steel

                                                           1.2% carbon steel

Microstructures of Non ferrous Alloys

Cast Brass

                                                                     Die cast alloy Al-Zn

                                             
                                                                         Gun Metal

                                                                         Rolled Brass

     
                                                                   Rolled Copper

                                                              Sand Cast Alloy(Al Si 12% modified)

Microstructures of Cast Iron

                                                            chilled cast iron

                                                               Feritic SG iron

                                                       Grey Cast Iron

                                                          Mealleable cast iron

                                                         pearlitic SG iron                    

                                                              white cast iron

notes of powder metallurgy

notes

Magnetic Refrigeration

ABSTRACT

 Ten years from now, it is highly likely that new room temperature magnetic refrigerators (RTMR) will be available to consumers as a more efficient and more environmentally safe alternative to conventional type refrigerators. Significantly, it has been estimated that improving our Nation’s refrigerators and freezers by just 4.2 percent would save an estimated power consumption equivalent to an average base-level power plant (479 million kWh).  Magnetic refrigeration cycle is estimated 50% more efficient than conventional fluid cycle, which would save the average power of 12 new base-level plants.  The magneto caloric effect (MCE) of rear earth alloys will be able to provide the adiabatic entropy change that is required achieving a refrigeration cycle at near room temperatures, which are presently achieved by the use of refrigerant fluids.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. This technique has been used for many years in cryogenic systems for producing further cooling in systems already cooled to temperatures of 4 Kelvin and lower. In England, a company called Cambridge Magnetic Refrigeration produces cryogenic systems based on the magneto caloric effect

The MCE is currently being explored to produce better refrigeration techniques, especially for use in spacecraft. There are still some thermal and magnetic hysteresis problems to be solved to make it really useful in industrial and household applications. This is a subject of current research.

Recent discovery has succeeded using commercial grade materials and permanent magnets on room temperatures to construct a magneto caloric refrigerator which promises wide use.

Key words: Refrigeration, Magnetic refrigeration, Magneto caloric effect (MCE),

1)    INTRODUCTION

Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or artificial process by which heat is dissipated. The process of artificially producing extreme cold temperatures is referred to as cryogenics. Magnetic refrigeration, or adiabatic demagnetization, is a cooling technology based on the magneto caloric effect. It is an intrinsic property of magnetic solids. The refrigerant is often 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. The magnetic refrigeration could be used in any possible application where cooling, heating or power generation is used today. Since it is only at an early stage of development, there are several technical and efficiency issues that should be analyzed. Magneto caloric effect (MCE) is the emission or absorption of heat in a magnetic material in response to a changing magnetic field.The magneto caloric refrigeration system is composed of pumps, electric motors, secondary fluids, heat exchangers of different types, magnets and magnetic materials. These processes are greatly affected by irreversibilities and should be adequately considered. Appliances using this method could have a smaller environmental impact if the method is perfected and replaces hydro fluorocarbon (HFCs) refrigerators which have considerable greenhouse effect. At present, however, the superconducting magnets that are used in the process have to themselves be cooled down to the temperature of liquid nitrogen, or with even colder, and relatively expensive, liquid helium. Considering these fluids have boiling points of 77.36 K and 4.22 K respectively, the technology is clearly not cost-efficient and efficient for home appliances, but for experimental, laboratorial, and industrial use only.The magnetic refrigeration based on MCE is becoming a promising technology to replace the conventional gas-compression/expansion technique.[a],[b].

 

2)    THE MAGNETO CALORIC EFFECT

The Magneto caloric 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. 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 (magneto caloric) 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. 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 magneto caloric 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 alloyed with nickel (PrNi5) which has strong magneto caloric effect. [b], [e].

 

 

3)    MAGNETO CALORIC CYCLE

Analogy between magnetic refrigeration and vapor cycle or conventional refrigeration:         H = externally applied magnetic field; Q = heat quantity; P = pressure; ΔTad = adiabatic temperature variation. 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).(Fig.1).The working material is the refrigerant, and starts in thermal equilibrium with the refrigerated environment. [b], [c].

a.                  Steps Involved In MCE Cycle

·         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's magnetic entropy 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 magneto caloric material and the coolant are separated (H=0).

·         Adiabatic demagnetization: The substance is returned to another adiabatic 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 transfers from thermal entropy to magnetic entropy i.e. 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, heat energy migrates into the working material (+Q).Once the refrigerant and refrigerated environment is in thermal equilibrium, the cycle continues.

b.                  Entropy(S) –Temperature (T) Diagram

 Magneto caloric effect (MCE) is the emission or absorption of heat in a magnetic material in response to a changing magnetic field. When a material is magnetized, the magnetic entropy ΔSm, is changed due to a change in the magnetic order of the material. Under adiabatic conditions, ΔSm must be compensated by the entropy associated with the lattice, resulting in a change in temperature of the material, ΔTad. The relation between ΔTad, ΔSm and magnetic properties of the material is illustrated in Fig.2.

 

4)    WORKING MATERIALS

The magneto caloric 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. 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. 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. Eventually paramagnetic salts become either diamagnetic or ferromagnetic, limiting the lowest temperature which can be reached using this method. (Fig.3)

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. [e].

5)    MAGNETIC FIELD DEVICE DESIGN

            Magnetic field and magnetic refrigerant can be assembled statically or dynamically. Statically the field is pulsed and power losses are high; dynamically uses either reciprocating or rotary movement, where extra power is required for the mechanics of the dynamic system. Conventional refrigerators compress a volatile gas and then permit it to rapidly expand, pulling heat from the surroundings. In contrast, the magnetic device exploits magnetically induced heating and cooling of a powder of the element gadolinium. The powder is stuffed in pockets inside the ring that carries it through the field of the permanent magnet. [d], [f].

It is clear from the fig. that as gadolinium enters a magnetic field and becomes magnetized; the material's atoms align, causing it to get hot. A fluid (red) carries that heat away. As the gadolinium exits the field, the atoms absorb heat from the recirculated fluid (blue) that chills a space. (Fig.4)

6)    ADVANTAGES OF MAGNETIC REFRIGERATION

a.       Magnetic refrigeration is environment friendly because there is no production of CFC, hazardous chemicals like NH3 (Ammonia) and greenhouse gases.

b.      The efficiency of magnetic refrigeration is 60% to 70% as compared to Carnot cycle. 

c.       The magnetic refrigeration consumes less power.

d.      Magnetic refrigeration is totally maintenance free & mechanically simple in construction.

e.       The C.O.P. (coefficient of performance) is very good as compared with conventional refrigeration.

7)    DISADVANTAGES OF MAGNETIC REFRIGERATION

The only disadvantage of magnetic refrigeration is that initial investment is more as compared with conventional refrigeration.

8)    APPLICATIONS OF MAGNETIC REFRIGERATION

a.       Magnetic refrigeration is currently being used to produce better refrigeration techniques,   especially for use in spacecraft.

b.      Magnetic refrigeration is used as magnetic refrigerator in house hold applications.

c.       Magnetic refrigeration is used as to produce very low temperature as 1 Kelvin.

d.      As the through magnetic refrigeration we can produce 20 Kelvin temperature which is liquefied point of Hydrogen gas so we can get liquid hydrogen gas from air as a fuel. Magnetic refrigeration is used in food preservation applications.

e.       Magnetic refrigeration is used to produce small as well as large capacity of crayocoolers. This crayocoolers have a lot many applications in Cryogenics.

9)    CASE STUDY- LIQUIFIED HYDROGEN

In 1997, the first near room temperature proof of concept magnetic refrigerator was demonstrated by Prof. Karl A. Gschneidner, Jr. by the Iowa State University at Ames Laboratory. [e].This event attracted interest worldwide that started developing new kinds of room temperature materials and magnetic refrigerator design.Liquid hydrogen could prove to be a perfect fuel, but first scientists and engineers must jump a few technological hurdles. One of the biggest hurdles, an efficient method of liquefying hydrogen has been eliminated by recent developments at Ames Laboratory. Scientists have developed a highly efficient magneto caloric material that makes magnetic refrigeration technology efficient enough to cheaply produce liquid hydrogen which is used in magnetic refrigerators.Gschneidner‘s latest discovery is a new class of alloys with significantly more cooling power than the best existing materials. The new material gadolinium has two to three times the magneto caloric effect of a typical ferromagnetic iron and a popular choice for low-temperature ranges. Additional work has revealed that Gd5Si2Ge2 is one of a family of compounds that exhibits a giant magneto caloric effect and whose ordering temperature can be tuned from 30 Kelvin (-405.4 F) to near room temperature (290 K or 62.6 F) by adjusting the ratio of silicon to germanium. (Fig.5)

Another factor helping to heat up the development of magnetic refrigeration technology is the recent ban on CFCs and other environmentally harmful substances. Magnetic refrigeration doesn't use CFCs and, in the case of the Astronautics model, water is used as the heat transfer fluid. Only antifreeze is added to allow the Astronautics unit to reach temperatures below 273 K, the freezing point of water. [g], [e].

 Large-scale applications include supermarket-sized refrigerators and freezers, air conditioning for large buildings, industrial chemical processing, and waste treatment.

10)        CONCLUSION

a.             Magnetic refrigeration has greater efficiency and would have beneficial effects on national power consumption. However, continued research in material sciences will be required to find a low cost material solution to magnetic refrigerant. 

b.            In addition, because permanent magnets account for a significant portion of the cost of prototypical systems, the development of higher performance and lower cost permanent magnet materials in the magnet industry will benefit the economics of magnetic refrigeration.

11)           REFERENCES

a.                   R. S. Khurmi, ‘Refrigeration & Air condition’, Eurasia Publishing house, Ramnagar,New Delhi 110055.

b.                  Stoecker and Jones, ‘Refrigeration and Air Conditioning’, Tata-McGraw Hill Publishers

c.                   Mathur, M.L., Mehta, F.S., ‘Thermal Engineering Vol II’

d.                  John Dieckmann, Member ASHRAE; Kurt Roth, Ph.D., Associate Member ASHRAE;and James Brodrick, Ph.D., Member ASHRAE

e.                   A b Karl Gschneidner, Jr. and Kerry Gibson (December 7, 2001); ‘Magnetic Refrigerator Successfully Tested’; Ames Laboratory News Release. Ames Laboratory. Retrieved on 2006-12-17.

f.                   http://www.sciencenews.org/articles/20020105/fob2.asp

g.                  http://www.external.ameslab.gov/news/Inquiry/fall97/bigchill.html