Czochralski process : to get hyper pure silicon (99.99999.....%)............

• The Czochralski process is a method of crystal growth used to obtain single crystals of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, platinum, silver, gold), salts, and synthetic gemstones. The process is named after Polish scientist Jan Czochralski, who discovered the method in 1916 while investigating the crystallization rates of metals.
• The most important application may be the growth of large cylindrical ingots, or boules, of single crystal silicon. Other semiconductors, such as gallium arsenide, can also be grown by this method, although lower defect densities in this case can be obtained using variants of the Bridgman-Stockbarger technique.
• High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted down in a crucible, which is usually made of quartz. Dopant impurity atoms such as boron or phosphorus can be added to the molten intrinsic silicon in precise amounts in order to dope the silicon, thus changing it into n-type or p-type extrinsic silicon. This influences the electronic properties of the silicon. A precisely oriented seed crystal, mounted on a rod, is dipped into the molten silicon. The seed crystal's rod is very slowly pulled upwards and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. Occurrence of unwanted instabilities in the melt can be avoided by investigating and visualizing the temperature and velocity fields during the crystal growth process.This process is normally performed in an inert atmosphere, such as argon, and in an inert chamber, such as quartz.
• Video for the process is here.......

A Free Book for Power Plant Engineering ..............

Here you go.........this book contains many topics that has been not covered in arora & domkundvar which is conventionally used by students...........
Power Plant Engineering

Chemical Machining : Advanced Manufacturing Method........

• Nontraditional machining processes are widely used to manufacture geometrically complex and precision parts for aerospace, electronics and automotive industries. There are different geometrically designed parts, such as deep internal cavities, miniaturized microelectronics and fine quality components may only be produced by nontraditional machining processes.
• Chemical Machining is a type of material removal process for the production of desired shapes and dimensions through selective or overall removal of material by controlled chemical attack with acids or alkalis often called as etchant solutions.
 
• There are mainly two types of Chemical Machining

1. Chemical Milling
2. Chemical Blanking

Chemical Milling

mainly used to produce shapes by selective or overall removal of metal parts from relatively large surface areas. The main purpose is to produce shallow cavities with complex profiles on plates, sheets, forgings, generally for the overall reduction of weight. This process has been used on a wide variety of metals with depths of metal removal as large as 12 mm.  Chemical milling entails four important steps:

1. Cleaning.
2. Masking.
3. Etching.
4. De-masking.

The stresses in the parts should be relieved in order to prevent warping after chemical milling. The surfaces are degreased and cleaned thoroughly to ensure both good adhesion of the masking material and uniform material removal.  Then the masking material is applied. Masking with tapes or paints (maskants) is a common practice, although elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and polystyrene) are also used. The maskant material should not react with the chemical reagent. If required, the maskant that covers various regions that require etching is peeled off by the scribe-and-peel technique.  The exposed surfaces are machined chemically with etchants, such as sodium hydroxide (for aluminium), solutions of hydrochloric and nitric acids (steels), or iron chloride (for stainless steels). Temperature control and agitation (stirring) during chemical milling is important in order to obtain a uniform depth from the material removed. After machining, the parts should be washed thoroughly to prevent further reactions with or exposure to any etchant residues. The rest of the masking material is removed and the part is cleaned and inspected. The masking material is unaffected by the reagent but usually is dissolved by a different type of solvent. Additional finishing operations may be performed on chemically milled parts. This sequence of operations can be repeated to produce stepped cavities and various contours. Schematic sketches of chemical milling process are shown in the Figure-1.

Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. Tank capacities for reagents are as large as 3.7 m X 15 m. This process is used to fabricate microelectronic devices and often is referred to as wet etching for these products.  Some surface damage may result from chemical milling because of preferential etching and intergranular attack, which adversely affect surface properties. The chemical milling of welded and brazed structures may result in uneven material removal. The chemical milling of castings may result in uneven surfaces caused by porosity and non-uniformity of the material.  With optimum time, temperature and solution control, accuracies of the range of plus or minus 0.01 mm can be achieved on relatively shallow depths of cut. The surface finish obtained may be around 5 microns. Aluminium alloys show better surface finish of the order of 1.6 microns. The metal removal rate on an aluminium component is reported to be about 140 cubic centimeters per minute.

Chemical Blanking
Chemical Blanking is similar to the blanking of sheet metals and it is applied to produce features, which penetrate through the thickness of the material, with the exception that the material is removed by chemical dissolution rather than by shearing. Typical applications for chemical blanking are the burr-free etching of printed-circuit boards, decorative panels, and thin sheet metal stampings, as well as the production of complex or small shapes. It is otherwise called as Chem-blanking, Photo forming, Photo fabrication, or Photo etching. In this process, the metal is totally removed from certain areas by chemical action. The process is used chiefly on the sheets and foils. This process can work almost any metal, however, it is not recommended for material thinner than 2 mm. A Schematic sketch of the chemical blanking process is shown in Figure-2

The work piece is cleaned, degreased and pickled by acid or alkalis. The cleaned metal is dried and photo resist material is applied to the work piece by dipping, whirl coating or spraying. It is then dried and cured. The technique of photography has been suitably employed to produce etchant resistant images in photo resist materials. This type of maskant is sensitive to light of a particular frequency, usually ultraviolet light, and not to room light. This surface is now exposed to the light through the negative, actually a photographic plate of the required design, just as in developing pictures. After exposure, the image is developed. The unexposed portions are dissolved out during the developing process exposing the bare metal. The treated metal is next put into a machine, which sprays it with a chemical etchant, or it may be dipped into the solution. The etching solution may be hydrofluoric acid (for titanium), or one of the several other chemicals. After 1 to 15 minute, the unwanted metal has been eaten away, and the finished part is ready for immediate rising to remove the etchant.  Chemical blanking by using photo resist maskants can suitably make printed circuit boards and blanking of intricate designs.
The advantages of this process are summarized below:

1. Very thin material (0.005 mm) can be suitably etched.
2. High accuracy of the order of plus or minus 0.015 mm can be maintained.
3. High production rate can be met by using automatic photographic technique. 

Manufacturing of Solar Cell & Solar Panels

Raw Materials

To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied to release the oxygen. The products are carbon dioxide and molten silicon. At this point, the silicon is still not pure enough to be used for solor cells and requires further purification.

Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous and boron to produce an excess of electrons and a deficiency of electrons respectively to make a semiconductor capable of conducting electricity. The silicon disks are shiny and require an anti-reflective coating, usually titanium dioxide.

The solar module consists of the silicon semiconductor surrounded by protective material in a metal frame. The protective material consists of an encapsulant of transparent silicon rubber or butyryl plastic (commonly used in automobile windshields) bonded around the cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight plastic cover on satellite arrays. The electronic parts are standard and consist mostly of copper. The frame is either steel or aluminum. Silicon is used as the cement to put it all together.

The Manufacturing
Process

• Purifying the silicon

1. The silicon dioxide of either quartzite gravel or crushed quartz is placed into an electric arc furnace. A carbon arc is then applied to release the oxygen. The products are carbon dioxide and molten silicon. This simple process yields silicon with one percent impurity, useful in many industries but not the solar cell industry.
2. The 99 percent pure silicon is purified even further using the floating zone technique. A rod of impure silicon is passed through a heated zone several times in the same direction. This procedure "drags" the impurities toward one end with each pass. At a specific point, the silicon is deemed pure, and the impure end is removed.

• Making single crystal silicon

1. Solar cells are made from silicon boules, polycrystalline structures that have the atomic structure of a single crystal. The most commonly used process for creating the boule is called the Czochralski method. In this process, a seed crystal of silicon is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and rotated, a cylindrical ingot or "boule" of silicon is formed. The ingot withdrawn is unusually pure, because impurities tend to remain in the liquid.

• Making silicon wafers

1. From the boule, silicon wafers are sliced one at a time using a circular saw whose inner diameter cuts into the rod, or many at once with a multiwire saw. (A diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.) Only about one-half of the silicon is lost from the boule to the finished circular wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular or hexagonal wafers are sometimes used in solar cells because they can be fitted together perfectly, thereby utilizing all available space on the front surface of the solar cell.
2. After the initial purification, the silicon is further refined in a floating zone process. In this process, a silicon rod is passed through a heated zone several times, which serves to 'drag" the impurities toward one end of the rod. The impure end can then be removed.
   Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is dipped into melted polycrystalline silicon. The seed crystal rotates as it is withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced out of the ingot.
3. The wafers are then polished to remove saw marks. (It has recently been found that rougher cells absorb light more effectively, therefore some manufacturers have chosen not to polish the wafer.

• Doping & Placing electrical contacts

1. The traditional way of doping (adding impurities to) silicon wafers with boron and phosphorous is to introduce a small amount of boron during the Czochralski process in step #3 above. The wafers are then sealed back to back and placed in a furnace to be heated to slightly below the melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the presence of phosphorous gas. The phosphorous atoms "burrow" into the silicon, which is more porous because it is close to becoming a liquid. The temperature and time given to the process is carefully controlled to ensure a uniform junction of proper depth.

   A more recent way of doping silicon with phosphorous is to use a small particle accelerator to shoot phosphorous ions into the ingot. By controlling the speed of the ions, it is possible to control their penetrating depth. This new process, however, has generally not been accepted by commercial manufacturers.

   Electrical contacts connect each solar cell to another and to the receiver of produced current. The contacts must be very thin (at least in the front) so as not to block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are vacuum-evaporated

   The anti-reflective coating

   • 9 Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon wafer. The most commonly used coatings are titanium dioxide and silicon oxide, though others are used. The material used for coating is either heated until its molecules boil off and travel to the silicon and condense, or the material undergoes sputtering. In this process, a high voltage knocks molecules off the material and deposits them onto the silicon at the opposite electrode. Yet another method is to allow the silicon itself to react with oxygen- or nitrogen-containing gases to form silicon dioxide or silicon nitride. Commercial solar cell manufacturers use silicon nitride.

   Encapsulating the cell

   • The finished solar cells are then encapsulated; that is, sealed into silicon rubber or ethylene vinyl acetate. The encapsulated solar cells are then placed into an aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover.

Friction Stir Welding : An Advance Welding Method...

• Friction-stir welding (FSW) is a solid-state joining process (meaning the metal is not melted during the process) and is used for applications where the original metal characteristics must remain unchanged as far as possible. This process is primarily used on alluminium and most often on large pieces which cannot be easily heat treated post weld to recover temper characteristics.
• It was invented and experimentally proven by Wayne Thomas and a team of his colleagues at The Welding Institute UK in December 1991. TWI holds a number of patents on the process, the first being the most descriptive.
• Principle of operation

In FSW, a cylindrical-shouldered tool, with a profiled threaded/unthreaded probe (nib or pin) is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces of sheet or plate material, which are butted together. The parts have to be clamped rigidly onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. The length of the nib is slightly less than the weld depth required and the tool shoulder should be in intimate contact with the work surface. The nib is then moved against the work, or vice versa.

Frictional heat is generated between the wear-resistant welding tool shoulder and nib, and the material of the work pieces. This heat, along with the heat generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without reaching the melting point (hence cited a solid-state process), allowing the traversing of the tool along the weld line in a plasticised tubular shaft of metal. As the pin is moved in the direction of welding, the leading face of the pin, assisted by a special pin profile, forces plasticised material to the back of the pin while applying a substantial forging force to consolidate the weld metal. The welding of the material is facilitated by severe plastic deformation in the solid state, involving dynamic recrystallization of the base material.