5.1.2. Chemical part of heat pipe manufacture

In the chemical part of the production, mechanical impurities and rust from the body of the heat pipe are first removed. This is followed by wet cleaning of the heat pipe components including cleaning with solutions, acids, and basic acids which are precisely determined for each type of material. The ultrasound cleaning, vacuuming, degassing, and passivation are processes that guarantee a high purity of the heat pipe material and thus contribute to longlasting failure free operation. Generally, two important goals are achieved by cleaning. The first goal is to ensure good wetting material of the heat pipe well by working. The second goal is to remove all particles of dirt because the presence of impurities in solid, liquid or gaseous form may have an adverse effect on the heat transfer ability of heat pipe. Small particles can inhibit the formation of capillary pressure in the wick structure. Machining or human hand grease may reduce the wettability of the wick structure. Oxides formed on the walls of the wick structure may also reduce the ability of the working fluid to wet the surface. It is also highly advisable to use an ultrasonic cleaner to clean the heat pipe material, as the ultrasound breaks down impurities firmly absorbed on the surface of metallic particles that cannot be removed in any other way. The cleaning of the heat pipe is repeated immediately before filling with the working fluid, after connecting the body with the end caps and the filling tube. After cleaning, the tube is degassed by heating to a higher temperature and vacuuming the interior. In the case of a wick heat pipe, it is necessary to remove the oxide layers from the wick structure by chemical cleaning (e.g., solvents).

### 5.1.3. Filling the heat pipe with the working fluid

The working fluid added into the heat pipe must be completely clean, free from all mechanical impurities and gases, as their trace residues can also react with the body material of the heat pipe and the formation of undesirable elements. Clean substances can be purchased without any problems at special chemical stores. However, even in pure liquids and solids, an incompressible gas may be present. These gases can be removed by repeated freezing and thawing cycles. The working fluid in the filling bottle can freeze using the liquid nitrogen or dry ice.

Filling process of each type of working fluid is happening under other conditions. The characteristic of the filling process depends on the state of the working fluid at ambient temperature. If the working fluid is at the room temperature in the gaseous state (cryogenic), the filling can be carried out via a gas container of high quality. Filling and closing process of liquid-metal heat pipes is appropriate to do in the vacuum chamber [37].

The filling of low-temperature heat pipes can be carried out at room temperature without the use of any protective atmosphere. Before filling the heat pipe, it is advisable sucking the air from it to ensure the removal of undesirable components contained in the materials which could be later shown as non-condensing components. In addition, under pressure, the working fluid naturally enters into the heat pipe, and thus the equilibrium state of the pure vapor and liquid phases at a lower pressure than atmospheric will be achieved [38].