10 Questions You Should to Know about What is high purity graphite?

Q.1 Is there an alternative to graphite foil for metallizing?

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 Supervac Graphite Foil- SV-GF

Ans. Graphite foil is highly conducting, flexible, compressible and relatively inexpensive. It serves to plug the gap between the evaporation boat and the copper clamp and so prevents any sparking that may take place. There is no alternate to graphite foil as no other material meets all these criterion.

Q.2 How should I determine what size of graphite tape I should be using?

Ans.  Size of graphite tape is defined by the thickness and width. Choice of thickness of graphite tape is based on condition (wear) of copper clamps. In new machines thickness of 0.2mm is normally used. As copper clamps keep wearing, thickness of graphite tape can be increased to 0.35mm and then 0.5mm.

Width of graphite tape is determine by the size of evaporation boats.

Q.3 Why use expensive Graphite Foil for a Metallizer?

 

Placement of Graphite Foil

Ans. Inside a vacuum chamber, Graphite foil is used as an electrically conducting gasket between the Evaporation Boat and the Copper Clamp. Here, it performs various important functions: 

1. It has to let very high electrical current pass without any resistance

2. It has to protect the costly Evaporation Boat from overheating and cracking and

3. It has to protect the Copper clamp from overheating and melting. 

For all these functions to be performed efficiently, the graphite foil must be of top quality – packing in denser amounts of graphite per sq.cm. as well as be very pure. Sadly, often for the sake of cost cutting or out of sheer ignorance, cheap, low quality, plumber grade graphite foil is used which leads to disastrous consequences. Plumber grade graphite foil cannot perform the aforementioned functions. This low quality graphite foil is not 100% pure since it is made with binders like resins, adhesives and rubber compounds which start releasing vapors when heated. These vapors bring down the vacuum and also cling to the substrate, thus affecting the quality of metallizing.

Q.4 What is the work of graphite foil in metallizing?

Ans. Graphite foil is placed between evaporation boat and copper clamp. It’s work is to fill any gaps between the two and provide smooth current flow. In absence of graphite foil there will be sparking between evaporation boat and clamp resulting in damage to clamp and boat and power wastage.

Q.5 How is graphite tape (used in metallizers) made?

Ans. A good quality graphite tape is prepared by exfoliating graphite powder and then compressing it under high pressure in a power press. This graphite tape does not have any binders and is therefore most suitable for use in Metallizers.

Q.6 Why is graphite foil used with ceramic evaporation boat in metallizing?

Ans. Evaporation boats get current from copper clamps and get heated up. Since surface of neither evaporation boat nor copper clamp is perfect so small gaps are left which cause sparking.

 Use of Graphite Foil

Graphite foil is put between boat and clamp. This fills any gaps. Further graphite foil is highly conductive so it prevents sparking and ensures copper heating of evaporation boats.

Q.7 What are the basic properties of graphite foil used in a metallizer?

Ans. A graphite foil used in metalizing should have following properties-

1. It should be made of high quality graphite powder not having any impurities.

2. Graphite foil should have been made without using any binders, resins etc. graphite content in this foil should be more than 99%.

3. Graphite foil should have high compressibility.

Q.8 What is the difference between special purpose graphite foil used in metallizing and normal graphite foil?

Ans. Graphite foil used in metallizer is of very high purity (more than 99%) as it is required to transmit very high current (up to 700 amp).

 Q. 9 What is the difference between crinkled graphite tape and plain graphite tape? Which is better?

Ans. Crinkled graphite tape is corrugated as shown below whereas plain graphite tape has a uniform cross section.

Plain graphite tape is better because of its uniform cross section it provides better contact between evaporation boats and copper clamps.

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Q.10 Some suppliers supply graphite foil in very long rolls of 30m/50m/100m whereas Supervac supplies in 500mm length. Why is it so?

 

 Supervac Graphite Foil- SV-GF

Ans. Graphite foils supplied by Supervac are made from 100% pure graphite powder which is pressed into the form of foil by a patented process of exfoliation.

Other suppliers who do not have this technology add binders to graphite powder so that it can be rolled into long rolls of 30/50/100 meters.

Needless to say binders affect the performance of graphite foil adversely.

graphite, mineral consisting of carbon. Graphite has a greasy feel and leaves a black mark, thus the name from the Greek verb graphein, “to write.”

Graphite has a layered structure that consists of rings of six carbon atoms arranged in widely spaced horizontal sheets. Graphite thus crystallizes in the hexagonal system, in contrast to diamond, another form of carbon, that crystallizes in the octahedral or tetrahedral system. Such pairs of differing forms of the same element usually are rather similar in their physical properties, but not so in this case. Graphite is dark gray to black, opaque, and very soft (with a Mohs scale hardness of 1.5), while diamond may be colorless and transparent and is the hardest naturally occurring substance (with a Mohs scale hardness of 10). Graphite is very soft because the individual layers of carbon atoms are not as tightly bound together as the atoms within the layer. It is an excellent conductor of heat and electricity. For detailed physical properties of graphite, see native element (table).

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carbon: Properties and uses

Before the discovery in 1779 that graphite when burned in air forms carbon dioxide, graphite was confused with both the metal lead and a superficially similar substance, the mineral molybdenite.

Graphite is formed by the metamorphosis of sediments containing carbonaceous material, by the reaction of carbon compounds with hydrothermal solutions or magmatic fluids, or possibly by the crystallization of magmatic carbon. It occurs as isolated scales, large masses, or veins in older crystalline rocks, gneiss, schist, quartzite, and marble and also in granites, pegmatites, and carbonaceous clay slates. Small isometric crystals of graphitic carbon (possibly pseudomorphs after diamond) found in meteoritic iron are called cliftonite.

Naturally occurring graphite is classified into three types: amorphous, flake, and vein. Amorphous is the most common kind and is formed by metamorphism under low pressures and temperatures. It is found in coal and shale and has the lowest carbon content, typically 70 to 90 percent, of the three types. Flake graphite appears in flat layers and is formed by metamorphism under high pressures and temperatures. It is the most commonly used type and has a carbon content between 85 and 98 percent. Vein graphite is the rarest form and is likely formed when carbon compounds react with hydrothermal solutions or magmatic fluids. Vein graphite can have a purity greater than 99 percent and is commercially mined only in Sri Lanka.

Graphite was first synthesized accidentally by Edward G. Acheson while he was performing high-temperature experiments on carborundum. He found that at about 4,150 °C (7,500 °F) the silicon in the carborundum vaporized, leaving the carbon behind in graphitic form. Acheson was granted a patent for graphite manufacture in 1896, and commercial production started in 1897. Since 1918 petroleum coke, small and imperfect graphite crystals surrounded by organic compounds, has been the major raw material in the production of 99 to 99.5 percent pure graphite.

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pencil with graphite

A pencil with graphite, the form of carbon that makes up the central core of a pencil.

Graphite is used in pencils, lubricants, crucibles, foundry facings, polishes, brushes for electric motors, and cores of nuclear reactors. Its high thermal and electrical conductivity make it a key part of steelmaking, where it is used as electrodes in electric arc furnaces. In the early 21st century, global demand for graphite has increased because of its use as the anode in lithium-ion batteries for electric vehicles. About 75 percent of graphite is mined in China, with significant amounts mined in Madagascar, Mozambique, and Brazil.