Glow discharge cathode layer

Using the aluminum sheet substrate as the cathode of a direct current (DQ glow discharge, cathodic plasma polymerization is carried out. Dealing with metal surfaces, cathodic plasma polymerization is the most practical means to provide the best corrosion protection (see Chapter 13). A primer is applied on the surface of the plasma polymer. The thickness of the plasma polymer is roughly 50 nm on average and that of the primer layer is about 30,000 nm (30 pm). Primers used included E-coat (electrolytic deposition of paint) and spray primers, but no top coat was applied in the study of corrosion protection. 

The energetically important parts of the discharge (cathode layer, dark space and negative glow) as well as the sample are inside the cathode cavity. The volatilization results from cathodic sputtering and/or thermal evaporation. This depends on the fact whether the whole cathode with its outer and inner wall is subjected to sputtering... 

The most common deposition technique is the glow-discharge decomposition (gdd) of volatile inorganic compounds - e.g., SiH4 to make a-Si H. Several variants are available where the ionized plasma is driven by a dc electric field (with the substrate on either the anode or the cathode), an ac field (60 Hz), an rf inductive, or rf capacitive (sometimes with a superposed dc bias). These techniques can be used in the presence of a magnetic field. The pressure of SiH4 is in the range 1 to 700 Pascal with the substrate at 200 300°C. Above 350°C, H2 evolves from the a-Si H layer ... 

Thus, almost the entire gap is charged positively. Dark-to-glow discharge transition at higher currents is due to growth of the positive space charge and distortion of the external electric field, which results in formation of the cathode layer. To describe the transition, the Maxwell equation can be used ... 

Voltage F, electric field A, and cathode layer length d are presented in Fig. 4-26 as functions of ctrrrent density, which is called the dimensionless current-voltage characteristic of a cathode layer. According to (4-37) ary cttrrent densities are possible in a glow discharge. In reality, a cathode layer prefers to operate at the only value of current density, the normal one jn (4-36), which corresponds to a minimttm of the cathode potential drop. It can be... 

Figure 4-26. Dimensionless eurrent-voltage cliaraeteristic and dimensionless parameters of a cathode layer in a glow discharge.

Normal Cathode Potential Drop, Normal Current Density, and Normal Thickness of Cathode Layer in Glow Discharges. Using (4-32) prove that the normal cathode potential drop does not depend in first approximation either on pressure or on gas temperature. Determine the dependence of normal current density, and normal thickness of the cathode layer on temperature at constant pressure and vice versa. 

One of the examples, the so-called cathode effect in A1 electrolysis, has been reported in 1966 [28]. This typical blockade phenomenon occurs at A1 cathode at very high current densities of about tens A cm . This effect shows itself in sharp increase of ohmic resistance at the cathode followed by arc or glow discharge. The early explanation suggested the formation of a layer of gaseous sodium. In our opinion, formation of dielectric solid film accounts better for this phenomenon. 

---