1)Luminescence (optical property)
Luminescence is defined broadly as the generation of light in excess of that radiated thermally. Man’s fascination with luminescence stems from when an otherwise invisible power is converted into visible light. The commercial importance of luminescence is ubiquitous, being manifest in lamps, displays, X-ray machines, etc.
Materials that generate luminescence are called phosphors. Commercial phosphors are mostly inorganic compounds prepared as powders (with grain sizes usually in the order of 2-20 µm) or thin films. The phosphor materials contain one or more impurity ions or activators (A), typically present in 0.01-100 mol % concentrations. The actual emission is generated on these activator ions.
Luminescence Science and Display Material, Ronda C., Srivastava A., The Electrochemical Society Interface, Spring 2006, Page: 55)
Luminescence(new)(better)
Some materials are capable of absorbing energy and then reemitting visible light in a phenomenon called luminescence. Photons of emitted light are generated from electron transitions in the solid. Energy is absorbed when an electron is promoted to an excited energy state; visible light is emitted when it falls back to a lower energy state if 1.8 eV < h < 3.1 eV. The absorbed energy may be supplied as higher-energy electromagnetic radiation (causing valence band–conduction band transitions, Figure 19.6a) such as ultraviolet light, or other sources such as high energy electrons, or by heat, mechanical, or chemical energy. Furthermore, luminescence is classified according to the magnitude of the delay time between absorption and reemission events. If reemission occurs for times much less than one second, the phenomenon is termed fluorescence; for longer times, it is called phosphorescence. A number of materials can be made to fluoresce or phosphoresce; these include some sulfides, oxides, tungstates, and a few organic materials. Ordinarily, pure materials do not display these phenomena, and to induce them, impurities in controlled concentrations must be added.
Luminescence has a number of commercial applications. Fluorescent lamps consist of a glass housing, coated on the inside with specially prepared tungstates or silicates. Ultraviolet light is generated within the tube from a mercury glow discharge, which causes the coating to fluoresce and emit white light. The picture viewed on a television screen is the product of luminescence. The inside of the screen is coated with a material that fluoresces as an electron beam inside the picture tube very rapidly traverses the screen. Detection of x-rays and -rays is also possible; certain phosphors emit visible light or glow when introduced into a beam of the radiation that is otherwise invisible.
Some p–n rectifying junctions, as described in Section 12.14, may also be used to generate visible light in a process termed electroluminescence. When a forwardbiased potential is applied across the device, electrons and holes will annihilate one another within the recombination region according to Equation 19.17. Under some circumstances the energy produced will appear as visible light. Such diodes that luminesce visible light are the familiar light-emitting diodes (LEDs), which are used for digital displays. The characteristic color of an LED depends on the particular semiconducting material that is used.
(Fundamentals of Materials Science and Engineering, William D. Callister, Jr. , p.312)
Polymorphic Transformations(new) (better)
Polymorphic Transformations are generally defined as those which involve a structural transition without a change in the chemical composition. Sometimes these transformations are also referred to as congruent processes. There are, however, several examples, such as the transformation of crystalline oxygen to crystalline ozone and transformations of position isomers, which satisfy the aforementioned definition of polymorphic transition, bu cannot even be consideredas phase transitions. This is because ozone and oxygen, in the phaserule sense, are two different substances (or components) which survived even the solid –liquid – vapour transitions while preserving their individuality. Similary each position isomer is an individual component and, therefore, isometric transtions cannot be considered as phase transitions. In the view of this, the definition of polymorphic Transformations needs to be restricted to transformations involveing phases with different crystal structures which are part of a single component system. In multicomponent metallic alloy and intermetallics, chemical composition-invariant crystallization is a good example in which the parent phase transforms to the product without allowing any partioning of the constituent elements ( or components) between the two phases. In this sense, the system behaves as if it is a single component system
(Phase Transformations: Examples from Titanium and Zirconium Alloys, Srikumar Banerjee, p.92)
3)Anodizing (Coating)
Although the previous processes are normally performed without electrolysis ,anodizing is an electrolytic treatment that produces a stable oxide layer on a metallic surface.Its most common applications are with aluminum and magnesium, but it is also applied to zinc, titanium and other less common metals.Anodized coatings are used primarily for decorative purposes;they also provide corrosion protection.
It is instructive to compare anodizing to electroplating since they are both electrolytic processes.Two differences stand out.1)In electrochemical plating,the workpart to be coated is the cathode in the reaction.By contrast, in anodizing the work is the anode ,whereas the processing tank is cathode.2)In electroplating the coating is grown by adhesion of ions of a second metal to the base metal surface.In anodizing the surface coating is formed through chemical reaction of the substrate metal into an oxide layer.
Anodized coatings usually range in thickness between 0.0025 and 0.075 mm.Dyes can be incorporated into the anodizing process to create a wide variety of colors;this is especially common in aluminium anodizing.Very thick coatings up to 0.25 mm can also be formed on aluminium by a special process called hard anodizing these coatings are noted for high resistance to wear and corrosion
(Fundamentals of modern manufacturing; materials,processes and systems,3rd edition, MikellP.Groover, p.674-675)20.21
Anodizing(new)
Anodizing is an electrochemical process, that is somewhat the reverse of electroplating,
which produces a conversion-type coating on aluminum that can improve corrosion and
wear resistance and impart a variety of decorative effects. If the workpiece is made the
anode of an electrolytic cell, instead of a plating layer being deposited on the surface, a
reaction progresses inward, increasing the thickness of the hard hexagonal aluminum
oxide crystals on the surface.The hardness depends on thickness, density, and porosity
of the coating, which are controlled by the cycle time and applied currents along with
the chemistry, concentration, and temperature of the electrolyte. The sul'face texture
very nearly duplicates the prefinishing texture, so a buffing prefinish produces a smooth,
lustrous coating while sand blasting produces a grainy or satiny coating.
The flow diagram in Figure 35-16 shows the anodizing process. Coating thicknesses
range from 0.1 mils to 0.25 mils. Note that the product dimensions will increase, however,
because the aluminum oxide coating occupies about twice the volume of the metal from
which it formed.
The nature of the developed coating is controlled by the electrolyte. If the oxide
coating is not soluble in the anodizing solution, it will grow until the resistance of the
oxide prevents current from flowing. The resultant coating, which is thin, nonporous,
and nonconducting,is used in a variety of electrical applications.
If the oxide coating is slightly soluble in the anodizing solution, dissolution competes
with oxide growth and a porous coating will be produced, where the pores provide
for continued current flow to the metal surface.As the coating thickens, the growth rate
decreases until it achieves steady state, where the growth rate is equal to the rate of dissolution.
This condition is determined by the specific conditions of the process, including
voltage, current density, electrolyte concentration, and electrolyte temperature.
Sulfuric, chromic, oxalic, and phosphoric acids all produce electrolytes that dissolve
oxide, with a sulfuric acid solution being the most common.
(Materials and Processes in Manufacturing, E. Paul DeGarmo,p.955)
- Sheet: 1100, 3003, and 6061
- Extrusions: 6061
- Casting alloys: 443 and 356
Porcelain Enameling (About Coating)(new)
Metals can also be coated with a variety of glassy, inorganic materials that impart resi
tance to corrosion and abrasion, decoratwe color, electrical insulation, or the abihty t
function in high-temperature environments. Multiple coats may be used, with the fi
or ground coat being selected to provide adhesion to the substrate and the cover co
to provide the surface characteristics. The material is usually applied in the form of am
ticomponent suspension or slurry (by dipping or spraying), which is then dried and fire
An alternative dry process uses electrostatic spraying of powder and subsequent firm
During the firing operation, which may require temperatures in the range of 800"
80UU°F, the coatlng materials melt,flow, and resolidify. Porcelain enamel is often foun
on the inner,petiorated tubs of many washing machines and may be used to Impart th
decorative exterior on cookpots and frying pans
(Materials and Processes in Manufacturing, E. Paul DeGarmo,p.958)
5)Thermal Spray Coatings (Coating) (new)
The thermal spray processes offer a means of applying a coating of high-performance material (metals, alloys, ceramics, intermetallics, cermets, carbides, or even plastics to more economical and more easily fabricated base metals. A wire or rod of the coating material is fed into a gas flame or arc, where it melts and becomes atomized by a stream of gas, such as argon, nitrogen, combustion gases, or compressed air.The gas stream propels the 0.01- to 0.05-mm (0.0004- to 0.002-in.) diameter particles toward the target surface, where they impact ("splat"), cool, and bond. Very little heat is transferred to the substrate, whose peak temperatures generally range from 100 to 250°C (200 to 500°F).As a result, thermal spraying does not induce undesirable metallurgical changes or excessive distortion, and coatings can be applied to thin or delicate targets or to heat sensitive materials such as plastics.The applied coating can range in thickness from 0.1 to 12 mm (0.004 to 0.5 in.).
Several of the thermal spray processes use adaptations of oxyfuel welding equipment. Figure 33-9 shows a schematic of an oxyacetylene metal spraying gun designed to utilize a solid wire feed.The flame melts the wire and a flow of compressed air disintegrates the molten material and propels it to the workpiece. An alternative type of oxyfuel gun uses material in the form of powder, which is gravity or pressure fed into the flame, where it is melted and carried by the flame gas onto the target.The powder feed permits the deposition of material that would be difficult to fabricate into wire such as cermets, oxides, and carbides. In addition, the droplet size is controlled by the powder, not by the factors that control atomization.
The lower temperatures and lower particle velocities of the oxyfuel deposition methods result in coatings with high porosity and low cohesive strength. An adaptation of the process known as high-velocity oxyfuel (HVOF) spraying propels the droplets with a supersonic stream of hot gas. Because the particles impact with high kinetic energies the resulting coating is dense and well bonded
(Materials and Processes in Manufacturing, E. Paul DeGarmo,p.898)
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