The mechanism and characteristics of the cathode are profound and extensive. As a layman, I can only share my own understanding. Interested parties are encouraged to seek out more information for their own study.

The phenomenon of thermionic emission from oxides was discovered in 1903, but research on it remains ongoing. Oxide cathodes are essentially electronic semiconductors. The operation of typical oxide cathodes is primarily based on excess barium atoms. Generally, during the vacuum pumping and high-temperature activation process, some of the oxygen atoms in the barium oxide in the cathode are reduced, resulting in excess barium atoms. Due to the high temperature, the valence electrons of the barium easily break free from their nuclei and become free electrons. This is a simple description of the operation of an oxide cathode.

The work function of an oxide cathode is related to the core metal; different core metals produce different work functions. Commonly used indirectly heated oxide cathodes are made of nickel. The work function of nickel oxide cathodes is 1.32 electron volts. Directly heated tubes often use tungsten filaments as the core metal, and the work function of tungsten is 1.36 electron volts. This shows that the cathode work function is solely related to the core metal and is unrelated to the tube type.

The properties of oxide cathodes are related to the reduction properties of the core metal. Typical nickel cathodes often contain trace amounts of magnesium, aluminum, silicon, tungsten, and other metals, which significantly impact cathode life and pulse characteristics. In directly heated cathodes, the aluminum and silicon content is often 100 times and 6 times higher than in indirectly heated nickel cathodes.

Compared to tungsten filament cathodes or atomic surface cathodes (thoriated tungsten, thorium carbide, etc.), oxide cathodes are more fragile, so we often see some problems.

1. Cathode poisoning.
Often, the extreme deterioration of cathodes in older tubes is largely due to cathode poisoning. Oxygen, carbon monoxide, carbon dioxide, water vapor, sulfides, halogens, hydrocarbons, and other compounds can all cause cathode poisoning. We often see old television damper tubes that, after long-term use under high voltage, have yellowed mica sheets and often poor cathode emission characteristics. One reason is that the mica partially decomposes under high voltage and high temperature, producing trace amounts of water vapor (often leaving mica debris in the tube), leading to cathode poisoning (of course, there are other causes).

2. Sparking.
In high-vacuum electron tubes, sparking caused by high currents is quite common. For example, some machines with direct-heated rectifiers have large filter capacitors. After powering on, sparking between the plate and cathode can sometimes occur. However, the operating voltage of the electron tube is far lower than the voltage at which static electricity can cause sparking, which is related to the oxide cathode.