Cathodic needle growth from Mo(CO)6 and Cr(CO)6 vapors at lower electric fields

A cathodic needle growth which is possibly associated with the temperature-field (T-F) electron emission is described. Experimental data disclosed that densely populated needle crystals exhibiting a dendritic configuration growth at the tip area of a pointed cathode when it is operated in an Mo(CO)₆ or Cr(CO)₆ atmosphere at a field strength just insufficient to draw field electrons. The needle growth occurs always at elevated temperatures of ∼600–∼700 K, indicating that it is triggered by T-F electrons emitted from the cathode tip. Needle crystals produced are not single crystals but composed of linearly packed micro-crystals, and those crystals obtainable from Mo(CO)₆ are a face-centered cubic phase of Mo₂C. Needles arising from Cr(CO)₆ have not been identified, but they are believed to be an unknown phase of chromium carbide.