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Wednesday, December 24, 2014

What is outgassing in vacuum science?

The generation of gas resulting from the desorption is known as outgassing. The outgassing constant is defined as the rate at which gas appears to emanate from unit area of surface, and is usually measured in units of Torr.Liter.Sec-1.Cm-2.     

If the temperature of the material is raised (baking), the outgasssing rate rises to a peak value.

Together with the acceleration of desorption, heating may also have the effect of causing activated chemisorption of physically adsorbed gas (in particular water vapour), which can then be desorbed only by prolonged heating at much high temperatures.


Chemi-adsorbed water vapor continues to be evolved at temperatures in excess of 300 degC. It should therefore appear that a degassing programme should begin with pumping at room temperature to remove physically adsorbed water vapor, before baking is commenced.

What is desorption?

When a material is placed in Vacuum, the gas which was previously adsorbed begins to desorb i.e. to leave the material.

The desorption is influenced by

1.       Pressure
2.       Temperature
3.       Shape of material
4.       Kind of its surface

The pressure has a basic influence on the desorption phenomenon since according to its tendency of increasing over or decreasing below the equilibrium , the phenomenon of sorption or that of desorption appears.

The temperature has a clear influence on desorption phenomena. Desorption is endothermic, thus it is accelerated by increase of temperature.

The shape of the material influences desorption either if the gas is adsorbed or absorbed.


If the gas is adsorbed, then only the amount of surface is the influencing factor, but if the gas has to diffuse from the interior of the material to the surface, then the third dimension “thickness” is also influencing the rate of desorption. 

ABOUT ELECTROLYTIC CAPACITORS

They provide more capacitance for their size than any other type. Electrolytics all have one thing in common, instead of usual plates separated by a dielectric, the electrolytic capacitor has a metallic anode coated with an oxide film. This outer covering is the dielectric, and a liquid electrolyte acts as a cathode. A second metallic conductor serves primarily as the connection to the liquid cathode, providing an external termination.
In actual practice, porous paper is wrapped around the anode and saturated with the electrolyte to eliminate the spillage problem.

There are two common types of electrolytic capacitors: a)  Aluminum  b) Tantalum

Both employ the same basic principle. The aluminum or Tantalum anode is covered with an oxide film. A suitable liquid or solid electrolyte is the cathode. The Aluminum type is by far the most popular because of its lower cost. The aluminum –oxide film has a very high resistance to current in one direction and has very low resistance to current in opposite direction. In other words, film acts as a dielectric in first instance and as a plate in second case. Because of this electrolytic are polarized. If the designated polarity is not observed, the oxide film on anode will breakdown and migrate to cathode connection, resulting in prompt failure of capacitor.

Electrolytics are described as belonging to one of the three basic families;

Polarized type
This type has one anode, the liquid cathode and a cathode connection. Polarity must be observed.

Semi polarized type
In many energy storage applications a certain amount of current reversal is often encountered. In such cases this type is recommended. In this type, the primary anode has a relatively thick oxide surface. The cathode connection now becomes the secondary anode with a thin oxide surface. Also liquid cathode has a slightly different chemical composition.

Non-polarized

This type is used in audio cross over networks and ac motor starting applications. Here there is a complete reversal of polarity; therefore, two anodes are required. The cathode connection now becomes a second, and equal, anode. Obviously size will be affected. Infact, the third type called nonpolarized is just twice as large as a polarized type of equivalent capacitance and voltage rating.

Monday, December 22, 2014

MICA CAPACITORS

Mica is popularly known as Isin glass. Important characteristics of Mica:

i) Its ability to operate at very high temperature (upto 500 oC).
ii) The material is almost totally inert and will not change with age, either chemically or physically.
iii) Mica is usable as a dielectric in its naturally state.
iv) It can be readily split into very thin sheets.

Mica capacitors are made by a method of depositing a thin layer of silver on each side of sheet of Mica was developed. This is done by a type of silver screen process. The silver is then fired in a furnace.

The principal advantage of Mica capacitor is its excellent degree of stability over a wide range of operating temperatures.

Mica capacitors are also among the best types to use where radio frequencies are involved.

Disadvantages of Mica capacitors:

i) They are relatively bulky when compared to other capacitors on a pure capacitance vs volume basis.


ii) The flat plates and method of construction lead to resonant frequency problems in some circuits.