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Anodic Glow Discharge II

Can Preparation

      From the previous photo you could see that surface preparation greatly affected the resulting oxide layer. I started to prep the cans in the following manner.

The clean can is now ready to receive an anodizing electrolyte and be conditioned. While cleaning is not essential to seeing the effect, it certainly makes the experiments more predictable and uniform.

Conditioning

     Conditioning can occur at any constant current where the overpotential is continuously rising. How much depends on a large number of factors. These experiments were all done in the range of 1 to 500 ma DC, with currents in the 10 milliamp range usually working for DC conditioning.

      It is also possible to 'pulse' condition anodes by exceeding their conditioning current for a short while. A variety of different effects can be seen by combining these modes. What works best strongly depends on the particulars of the experiment.

Electrolytes

     Broadly speaking, electrolytes can be divided into two categories. Electrolytes such as citric and boric acid work well in saturated solutions, and the dominant anode reaction is the generation of the oxide layer. Oxygen generation at the anode is minimal. The resulting layer deposits smoothly over a broad range of current densities, and the cell impedance is quite high. As a results, high voltages are required to get enough current density to see substantial glow.

     Electrolytes such as oxalic acid or sodium pyrophosphate allow for a great deal of oxygen generation at the anode, consequently the impedance is low and a porous oxide layer is formed due to the outgassing. It is very hard to condition a cell to higher voltages, but the glow is more intense at lower voltages due to the increase in current density.

     Making the solution more dilute helps raise the voltage that can be achieved by conditioning, to a point. In some cases (such as citric acid) the opposite seemed true, for example going from 10% to saturated gave much better results. If the goal is efficient generation of glow, low voltage electrolytes may prove superior, as their overpotential is much lower for a given current density.

Citric Acid Electrolyte

     Here is a typical high current high voltage discharge after 50% citric acid anodization. The resulting thick film of oxide gives the glow a soft uniform lemony green color.

Note the white speckles concentrated on the rim and scattered across the oxide layer, these are the precursors to the arcing mode. Arcing will start at that luminous crescent on the rim, and eventually overtakes the glow as the current sink.

Sodium Pyrophosphate Electrolyte

     Electrolyte chemistry plays a role in the color of the glow. Here is a typical high current low voltage discharge in a dilute sodium pyrophosphate solution. The orange color grows more intense as anodizing continues.

I found it hard to control temperature at these current densities, and some electrolytes require massive currents to passivate the anode. Better heat sinking is called for.

VI Curve of the anodic glow discharge

     A anode was formed by conditioning in 50% citric acid, washed in distilled water, and the following 'virgin' VI curve was measured in a fresh 50% citric acid electrolyte.

This curve is typical for anodic discharge experiments and is somewhat indicative of an electrochemical reaction described by the Tafel equation, where some fixed potential is required to initiate the reaction ( in this case around 75 to 100 volts ) and an overpotential is required to push more current through the cell and increase the reaction. With these cells the whole overpotential is at the anode and is often very substantial. It is also the case that the Tafel a and b parameters can only characterize a particular discharge experiment, as conditioning can affect both parameters.

     It is clear from the graph that the glow is connected to current density. Some intermediate chemical species must be forming on the aluminum under anodizing conditions which causes the glow. In the case of high current density electrolytes a substantial amount of this intermediate species (or the resulting oxide) is redissolving in the electrolyte, hence the difficulty in conditioning and the persistence of a strong glow. To maintain a glow at a constant current in the case where the oxide layer is strong and stays formed requires an ever increasing voltage.

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