Lecture notes on material science pdf

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Lecture notes files. Caroline Ross. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite.

The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling. This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite- Austenite region, line A1on the diagram.

This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air. Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.

Stress Relief Anneal is used to reduce residual stresses in large castings, welded parts and cold- formed parts. Such parts tend to have stresses due to thermal cycling or work hardening. Hold the temperature for a prolonged time and follow by fairly slow cooling. All these methods result in a structure in which all the Cementite is in the form of small globules spheroids dispersed throughout the ferrite matrix.

This structure allows for improved machining in continuous cutting operations such as lathes and screw machines. Spheroidization also improves resistance to abrasion.

Tempering is a process done subsequent to quench hardening. Quench-hardened parts are often too brittle. This brittleness is caused by a predominance of Martensite. This brittleness is removed by tempering.

Tempering results in a desired combination of hardness, ductility, toughness, strength, and structural stability. Tempering is not to be confused with tempers on rolled stock-these tempers are an indication of the degree of cold work performed.

The mechanism of tempering depends on the steel and the tempering temperature. The prevalent Martensite is a somewhat unstable structure. When heated, the Carbon atoms diffuse from Martensite to form a carbide precipitate and the concurrent formation of Ferrite and Cementite, which is the stable form.

Tool steels for example, lose about 2 to 4 points of hardness on the Rockwell C scale. Even though a little strength is sacrificed, toughness as measured by impact strength is increased substantially. Springs and such parts need to be much tougher — these are tempered to a much lower hardness. Tempering is done immediately after quench hardening. In this region a softer and tougher structure Troostite is formed. This has less strength than Troostite but more ductility and toughness.

Heating in a bath also ensures that the entire part has the same temperature and will undergo the same tempering. Regardless of the bath, gradual heating is important to avoid cracking the steel. After reaching the desired temperature, the parts are held at that temperature for about 2 hours, then removed from the bath and cooled in still air. Hardening Hardness is a function of the Carbon content of the steel. Hardening of a steel requires a change in structure from the body-centered cubic structure found at room temperature to the face- centered cubic structure found in the Austenitic region.

The steel is heated to Autenitic region. When suddenly quenched, the Martensite is formed. This is a very strong and brittle structure. When slowly quenched it would form Austenite and Pearlite which is a partly hard and partly soft structure. When the cooling rate is extremely slow then it would be mostly Pearlite which is extremely soft. Usually when hot steel is quenched, most of the cooling happens at the surface, as does the hardening. This propagates into the depth of the material.

Alloying helps in the hardening and by determining the right alloy one can achieve the desired properties for the particular application. The water adjacent to the hot steel vaporizes, and there is no direct contact of the water with the steel.

This slows down cooling until the bubbles break and allow water contact with the hot steel. As the water contacts and boils, a great amount of heat is removed from the steel. With good agitation, bubbles can be prevented from sticking to the steel, and thereby prevent soft spots. Water is a good rapid quenching medium, provided good agitation is done.

However, water is corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking. Salt Water: Salt water is a more rapid quench medium than plain water because the bubbles are broken easily and allow for rapid cooling of the part.

However, salt water is even more corrosive than plain water, and hence must be rinsed off immediately. Oil: Oil is used when a slower cooling rate is desired. Since oil has a very high boiling point, the transition from start of Martensite formation to the finish is slow and this reduces the likelihood of cracking. Oil quenching results in fumes, spills, and sometimes a fire hazard. This leads to a supersaturated solid solution that remains stable metastable due to the low temperatures, which prevent diffusion.

If the process is continued for a very long time, eventually the hardness decreases. This is called over aging. The precipitates form when the solubility limit is exceeded. Precipitation hardening is also called age hardening because it involves the hardening of the material over a prolonged time.

Case Hardening Case hardening produces a hard, wear-resistant sur- face or case over a strong, tough core. The principal forms of casehardening are carburizing, cyaniding, and nitriding.

Only ferrous metals are case-hardened. Case hardening is ideal for parts that require a wear-resistant surface and must be tough enough interally to withstand heavy loading. The steels best suited for case hardening are the low-carbon and low-alloy series. When high-carbon steels are case hardened, the hardness penetrates the core and causes brittleness.

In case hardening, you change the surface of the metal chemically by introducing a high carbide or nitride content. The core remains chemically unaffected. When heat-treated, the high-carbon surface responds to hardening, and the core toughens.

This results in a carburized steel that has a high-carbon surface and a low-carbon interior. When the carburized steel is heat-treated, the case be- comes hardened and the core remains soft and tough. Two methods are used for carburizing steel. One method consists of heating the steel in a furnace containing a carbon monoxide atmosphere.

The other method has the steel placed in a container packed with charcoal or some other carbon-rich material and then heated in a furnace. To cool the parts, you can leave the container in the furnace to cool or remove it and let it air cool. In both cases, the parts become annealed during the slow cooling. The depth of the carbon penetration depends on the length of the soaking period. Cyaniding This process is a type of case hardening that is fast and efficient.

Preheated steel is dipped into a heated cyanide bath and allowed to soak. Upon removal, it is quenched and then rinsed to remove any residual cyanide. This process produces a thin, hard shell that is harder than the one produced by carburizing and can be completed in 20 to 30 minutes vice several hours.

The major drawback is that cyanide salts are a deadly poison. Nitriding This case-hardening method produces the hardest surface of any of the hardening processes. It differs from the other methods in that the individual parts have been heat-treated and tempered before nitriding. The parts are then heated in a furnace that has an ammonia gas atmosphere.

No quenching is required so there is no worry about warping or other types of distortion. This process is used to case harden items, such as gears, cylinder sleeves, camshafts and other engine parts, that need to be wear resistant and operate in high-heat areas. Flame Hardening Flame hardening is another procedure that is used to harden the surface of metal parts.

When you use an oxyacetylene flame, a thin layer at the surface of the part is rapidly heated to its critical temperature and then immediately quenched by a combination of a water spray and the cold base metal. Whether the process is manual or mechanical, a close watch must be maintained, since the torches heat the metal rapidly and the temperatures are usually determined visually. Flame hardening may be either manual or automatic. Automatic equipment produces uniform results and is more desirable.

Most automatic machines have variable travel speeds and can be adapted to parts of various sizes and shapes. The size and shape of the torch depends on the part. The torch consists of a mixing head, straight extension tube, degree extension head, an adjustable yoke, and a water-cooled tip.

Practically any shape or size flame- hardening tip is available. Tips are produced that can be used for hardening flats, rounds, gears, cams, cylinders, and other regular or irregular shapes. In hardening localized areas, you should heat the metal with a standard hand-held welding torch.

Adjust the torch flame to neutral for normal heating; however, in corners and grooves, use a slightly oxidizing flame to keep the torch from sputtering. You also should particularly guard against overheating in comers and grooves. If dark streaks appear on the metal surface, this is a sign of overheating, and you need to increase the distance between the flame and the metal. For the best heating results, hold the torch with the tip of the inner cone about an eighth of an inch from the surface and direct the flame at right angles to the metal.

Sometimes it is necessary to change this angle to obtain better results; however, you rarely find a deviation of more than 30 degrees. Regulate the speed of torch travel according to the type of metal, the mass and shape of the part, and the depth of hardness desired. In addition, you must select the steel according to the properties desired.

Select carbon steel when surface hardness is the primary factor and alloy steel when the physical properties of the core are also factors. Plain carbon steels should contain more than 0. For water quench- ing, the effective carbon range is from 0. Parts with a carbon content of more than 0. The surface hardness of a flame-hardened section is equal to a section that was hardened by furnace heating and quenching. The decrease in hardness between the case and the core is gradual.

Since the core is not affected by flame hardening, there is little danger of spalling or flaking while the part is in use. Thus flame hardening produces a hard case that is highly resistant to wear and a core that retains its original properties. Flame hardening can be divided into five general methods: stationary, circular band progressive, straight- line progressive, spiral band progressive, and circular band spinning. The gradient can be a compositional gradient, an electric or magnetic gradient, or stress gradient.

Many reactions in solids and liquids are diffusion dependent. Diffusion is very important in many industrial and domestic applications. From an atomic perceptive, diffusion is a step wise migration of atoms from one lattice position to another. Atom diffusion can occur by the motion of vacancies vacancy diffusion or impurities impurity diffusion. The energy barrier is that due to nearby atoms which need to move to let the atoms go by.

This is more easily achieved when the atoms vibrate strongly, that is, at high temperatures. There is a difference between diffusion and net diffusion. In a homogeneous material, atoms also diffuse but this motion is hard to detect. This is because atoms move randomly and there will be an equal number of atoms moving in one direction than in another. In inhomogeneous materials, the effect of diffusion is readily seen by a change in concentration with time.

In this case there is a net diffusion. Net diffusion occurs because, although all atoms are moving randomly, there are more atoms moving in regions where their concentration is higher. Steady state diffusion means that J does not depend on time. The concentration gradient is often called the driving force in diffusion but it is not a force in the mechanistic sense. The minus sign in the equation means that diffusion is down the concentration gradient.

Nonsteady-State Diffusion This is the case when the diffusion flux depends on time, which means that a type of atoms accumulates in a region or that it is depleted from a region which may cause them to accumulate in another region. Factors that influence diffusion As stated above, there is a barrier to diffusion created by neighboring atoms that need to move to let the diffusing atom pass. Thus, atomic vibrations created by temperature assist diffusion.

Pure Cu is soft and malleable, difficult to machine. Brass is an alloy with Zn. Bronzes contain tin, aluminum, silicon or beryllium. More than copper- base alloys are recognized. It has atomic number It is the first element of group 12 of the periodic table. Zinc is the 24th most abundant element in the Earth's crust and has five stable isotopes. The most common zinc ore is sphalerite zinc blende , a zinc sulfide mineral.

The largest mineable amounts are found in Australia, Asia, and the United States. Zinc production includes froth flotation of the ore, roasting, and final extraction using electricity electrowinning. Brass, which is an alloy of copper and zinc, has been used since at least the 10th century BC in Judea and by the 7th century BC in Ancient Greece. Zinc metal was not produced on a large scale until the 12th century in India and was unknown to Europe until the end of the 16th century.

The mines of Rajasthan have given definite evidence of zinc production going back to 6th century BC. To date, the oldest evidence of pure zinc comes from Zawar, in Rajasthan, as early as the 9th century AD when a distillation process was employed to make pure zinc. Alchemists burned zinc in air to form what they called "philosopher's wool" or "white snow".

Galvanization, which is the coating of iron or steel to protect the metals against corrosion, is the most familiar form of using zinc in this way. Zinc is more reactive than iron or steel and thus will attract almost all local oxidation until it completely corrodes away. A protective surface layer of oxide and carbonate forms as the zinc corrodes. The zinc is applied electrochemically or as molten zinc by hot-dip galvanizing or spraying.

Galvanization is used on chain-link fencing, guard rails, suspension bridges, light posts, metal roofs, heat exchangers, and car bodies.

The relative reactivity of zinc and its ability to attract oxidation to itself makes it an efficient sacrificial anode in catholic protection CP. For example, cathodic protection of a buried pipeline can be achieved by connecting anodes made from zinc to the pipe. Zinc acts as the anode negative terminus by slowly corroding away as it passes electric current to the steel pipeline. Zinc is also used to catholically protect metals that are exposed to sea water from corrosion.

A zinc disc attached to a ship's iron rudder will slowly corrode while the rudder stays unattacked. Other similar uses include a plug of zinc attached to a propeller or the metal protective guard for the keel of the ship.

Powdered zinc is used in this way in alkaline batteries and sheets of zinc metal form the cases for and act as anodes in zinc—carbon batteries. The zinc-cerium redox flow battery also relies on a zinc-based negative half- cell. Brass is generally more ductile and stronger than copper and has superior corrosion resistance. These properties make it useful in communication equipment, hardware, musical instruments, and water valves. Other widely used alloys that contain zinc include nickel silver, typewriter metal, soft and aluminium solder, and commercial bronze.

Zinc is the primary metal used in making American one cent coins since The zinc core is coated with a thin layer of copper to give the impression of a copper coin.

In , 33, tonnes 36, short tons of zinc were used to produce These alloys are marketed under the name Zamak. An example of this is zinc aluminium. The low melting point together with the low viscosity of the alloy makes the production of small and intricate shapes possible.

The low working temperature leads to rapid cooling of the cast products and therefore fast assembly is possible. This superplasticity of the alloy allows it to be molded using die casts made of ceramics and cement.

Similar alloys with the addition of a small amount of lead can be cold-rolled into sheets. As a dense, inexpensive, easily worked material, zinc is used as a lead replacement. In the wake of lead concerns, zinc appears in weights for various applications ranging from fishing to tire balances and flywheels.

Cadmium zinc telluride CZT is a semi conductive alloy that can be divided into an array of small sensing devices. These devices are similar to an integrated circuit and can detect the energy of incoming gamma ray photons.

When placed behind an absorbing mask, the CZT sensor array can also be used to determine the direction of the rays. Other industrial uses Zinc oxide is used as a white pigment in paints. Roughly one quarter of all zinc output in the United States , is consumed in the form of zinc compounds; a variety of which are used industrially.

Zinc oxide is widely used as a white pigment in paints, and as a catalyst in the manufacture of rubber. The zinc zinc-oxide cycle is a two step thermochemical process based on zinc and zinc oxide for hydrogen production.

Zinc chloride is often added to lumber as a fire retardant and can be used as a wood preservative. It is also used to make other chemicals. Crystals of ZnS are used in lasers that operate in the mid-infrared part of the spectrum.

Zinc sulfate is a chemical in dyes and pigments. Zinc pyrithione is used in antifouling paints. Zinc powder is sometimes used as a propellant in model rockets.

This produces zinc sulfide, together with large amounts of hot gas, heat, and light. Zinc sheet metal is used to make zinc bars. Zn, the most abundant isotope of zinc, is very susceptible to neutron activation, being transmuted into the highly radioactive Zn, which has a half-life of days and produces intense gamma radiation.

Because of this, Zinc Oxide used in nuclear reactors as an anti-corrosion agent is depleted of Zn before use, this is calleddepleted zinc oxide. For the same reason, zinc has been proposed as a salting material for nuclear weapons cobalt is another, better-known salting material. A jacket of isotopically enriched Zn would be irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, forming a large amount of Zn significantly increasing the radioactivity of the weapon's fallout.

Such a weapon is not known to have ever been built, tested, or used. Zn is also used as a tracer to study how alloys that contain zinc wear out, or the path and the role of zinc in organisms. Chromium Chromium is a chemical element which has the symbol Cr and atomic number It is the first element in Group 6. Chromium oxide was used by the Chinese in the Qin dynasty over 2, years ago to coat metal weapons found with the Terracotta Army.

Chromium was discovered as an element after it came to the attention of the western world in the red crystalline mineral crocoite lead II chromate , discovered in and initially used as a pigment. Louis Nicolas Vauquelin first isolated chromium metal from this mineral in Since Vauquelin's first production of metallic chromium, small amounts of native free chromium metal have been discovered in rare minerals, but these are not used commercially. Instead, nearly all chromium is commercially extracted from the single commercially viable orechromite, which is iron chromium oxide Chromite is also now the chief source of chromium for chromium pigments.

Applications The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. For its formation, ferrochromium is added to the molten iron.

Also nickel-based alloys increase in strength due to the formation of discrete, stable metal carbide particles at the grain boundaries. For example, Inconel contains Because of the excellent high-temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials. The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability.

A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or dichromate solutions. To prevent the energy-consuming change in oxidation state, the use of chromium III sulfate is under development, but for most applications, the established process is used.

This passivation and the self-healing properties by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating methodBecause of environmental and health regulations on chromates, alternative coating method are under development. Anodizing of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as electrolyte in the solution.

During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers. The high toxicity of Cr VI compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium or at least a change to less toxic chromium III compounds.

Dye and pigment The mineral crocoite lead chromate PbCrO4 was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, chrome yellow was, together with cadmium yellow, one of the most used yellow pigments.

Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics. Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leaves. Synthetic ruby and the first laser Natural rubies are corundum aluminum oxide crystals that are colored red the rarest type due to chromium III ions other colors of corundum gems are termedsapphires.

For example, chromated copper arsenate CCA is used in timber treatment to protect wood from decay fungi, wood attacking insects, including termites, and marine borers Refractory material The high heat resistivity and high melting point makes chromite and chromium III oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metals. In these applications, the refractory materials are made from mixtures of chromite and magnesite.

The use is declining because of the environmental regulations due to the possibility of the formation of chromium VI. Bronze is an alloy consisting primarily of copper, usually with tin as the main additive. It is hard and tough, and it was so significant in antiquity that the Bronze Age was named after the metal. They contain only one phase, with face-centered cubic crystal structure. Alpha-beta brasses are usually worked hot. Aluminium brass contains aluminium, which improves its corrosion resistance.

Red brass is both an American term for the copper-zinc-tin alloy known as gunmetal, and an alloy which is considered both a brass and a bronze.

It fairs well in the marine enviroment. It is also resistant to corrosion and therefore good for use in the marine enviroment. Rather than Phosphor being the main alloy metal, it is used during manufacture to purify the melt and create a purer stronger type of bronze. Usually not more than 0.

It is very corrosion resistant and therefore good in the marine environment. Typical compostion: Copper It is not considered to be brass due to the amount of zinc being small. It has widespread use as a valve and through hull fitting material due to it's good seawater resistance. It does not de-zincify in seawater despite having a zinc content. It is good for use in the marine environment. Bronze is still commonly used in ship propellers and submerged bearings.

In the 20th century, silicon was introduced as the primary alloying element, creating an alloy with wide application in industry and the major form used in contemporary statuary. Sculptors may prefer silicon bronze because of the ready availability of silicon bronze brazing rod, which allows color-matched repair of defects in castings.

Aluminium is also used for the structural metal aluminium bronze. It is also widely used for cast bronze sculpture. Many common bronze alloys have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mold.

Bronze parts are tough and typically used forbearings, clips, electrical connectors and springs. Bronze also has very low metal-on-metal friction, which made it invaluable for the building of cannon where iron cannonballs would otherwise stick in the barrel. It is still widely used today for springs, bearings, bushings, automobile transmission pilot bearings, and similar fittings, and is particularly common in the bearings of small electric motors.

Phosphor bronze is particularly suited to precision-grade bearings and springs. It is also used in guitar and piano strings. Unlike steel, bronze struck against a hard surface will not generate sparks, so it along with beryllium copper is used to make hammers, mallets, wrenches and other durable tools to be used in explosive atmospheres or in the presence of flammable vapors.

Bronze is used to make bronze wool for woodworking applications where steel wool would discolor oak. The materials are processed to achieve desirable properties to maximize bearing performance and life. The materials described here are the most commonly used. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization.

Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field that reduces the overall field within the dielectric itself.

If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axes align to the field. The study of dielectric properties concerns storage and dissipation of electric and magnetic energy in materials. Dielectrics are important for explaining various phenomena in electronics, optics, and solid-state physics.

Dielectric strength is measured through the thickness of the material taking care to avoid surface effects and is normally expressed as a voltage gradient volts per unit length. The value of dielectric strength for a specimen is also influenced by its temperature and ambient humidity, by any voids or foreign materials in the specimen, and by the conditions of test, so that it is often difficult to compare data from different sources.

Test variables include electrode configuration and specimen geometry, and the frequency and rate of application of the test voltage. Magnetic Properties Diamagnetism is a very weak form of magnetism that is nonpermanent and persists only while an external field is being applied. It is induced by a change in the orbital motion of electrons due to an applied magnetic field.

The magnitude of the induced magnetic moment is extremely small and in a direction opposite to that of the applied field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic. Paramagnetic material is one whose atoms do have permanent dipole moments, but the magic of ferromagnetism is not active.

If a magnetic field is applied to such a material, the dipole moments try to line up with the magnetic field, but are prevented from becoming perfectly aligned by their random thermal motion. Because the dipoles try to line up with the applied field, the susceptibilities of such materials are positive, but in the absence of the strong ferromagnetic effect, the susceptibilities are rather small, say in the range to.

When a paramagnetic material is placed in a strong magnetic field, it becomes a magnet, and as long as the strong magnetic field is present, it will attract and repel other magnets in the usual way. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum. Ferromagnetism: Certain metallic materials possess a permanent magnetic moment in the absence of an external field, and manifest very large and permanent magnetizations. These are the characteristics of ferromagnetism, and they are displayed by the transition metals iron, cobalt, nickel, and some of the rare earth metals.

Permanent magnetic moments in ferromagnetic materials result from atomic magnetic moments due to electron spinuncancelled electron spins as a consequence of the electron structure.

There is also an orbital magnetic moments contribution that is small in comparison to the spin moment. Furthermore, in a ferromagnetic material, coupling interactions cause net spin magnetic moments of adjacent atoms to align with one another, even in the absence of an external field.

The maximum possible magnetization or saturation magnetization Ms of a ferromagnetic material represents the magnetization that results when all the magnetic diploes in a solid piece are mutually aligned with the external field; there is also a corresponding saturation flux density Bs.

Iron, nickel, and cobalt are examples of ferromagnetic materials. Antiferromagnetism: This phenomenon of magnetic moment coupling between adjacent atoms or ions occurs in materials other than those that are ferromagnetic.

In one such group, this coupling results in an antiparallel alignment; the alignment of the spin moments of neighbouring atoms or ions in exactly opposite directions is termed antiferromagentism. Manganese Oxide MnO is one such material that displays this behavior. Manganese oxide is a ceramic material that is ionic in character, having both Mn and O ions. No net magnetic moment is associated with O ions, since there is a total cancellation of both spin and orbital moments.

However, the Mn ions possesses s nrt magnetic moment that is Material predominantly of spin origin. These Mn ions are arrayed in the crystal structure such that the moments of adjacent ions are antiparallel. Obvioulsy, the opposing magnetic moments cancel one another and as a consequence, the solid as a whole possesses no net magnetic moment. The macroscopic magnetic characteristics of ferromagnets and ferrimagents are similar; the distinction lies in the source of the net magnetic moments.

The net ferrimagentic moment arises from the incomplete cancellation of spin moments. Cu - moment 0. Hard Magnetic Materials Hard magnets, also referred to as permanent magnets, are magnetic materials that retain their magnetism after being magnetised. These magnets were permanent magnets due to the pinning of domain walls by dislocations and inclusions.

The movement of dislocations within a material is often hindered by the same factors that effect the motion of domain walls and as a consequence these steels are mechanically very hard and are the origin of the term hard magnetic.

These magnets had an energy product of approximately 8kJm- Soft Magnetic Materials Soft magnetic materials are those materials that are easily magnetised and demagnetised.

They typically have intrinsic coercivity less than Am