Aquadag

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Oscilloscope tube with clearly visible Aquadag layer

Aquadag is an aqueous suspension of colloidal graphite in gel-like liquids. This suspension was developed in the autumn of 1906 by the American industrialist Edward Goodrich Acheson , where he initially suggested its use as a lubricant. Acheson said that he patented the development in 23 countries and had to enter the brand name Aquadag where Aquadag an acronym for Aqua d eflocculated a Cheson g is raphite. It was manufactured by Acheson Industries , which became a subsidiary of Imperial Chemical Industries (ICI) in 1998 and which is now part of Henkel . Aspen states that the ingredients are agar , water , graphite powder and ammonia , which are mixed in a ball mill . Coatings obtained with the help of Aquadag are z. B. used in vacuum technology when electrically conductive , temperature-insensitive coatings with a low tendency to secondary electron emission are required.

use

In the manufacture of cathode ray tubes, Aquadag was applied to the inner surface of part of the rear glass bulb in order to obtain electrically conductive layers which are supposed to absorb stray electrons and shield external electric fields . During the heating of the assembled tubes, the organic components disintegrate and are sucked off with the vacuum pump , while a pure, well-adhering and weakly conductive graphite layer remains on the glass bulb.

A second layer of Aquadag was often applied to the outside rear of the cathode ray tube, especially with television picture tubes: While the inner layer is conductively connected to the high-voltage anode, the outer layer, like the cathode, is at ground level. In this way, the two layers can act like a very high-voltage-resistant plate capacitor , which smooths the high voltage and thus compensates for irregularities in the high-voltage output of the flyback transformer . An ordinary capacitor with the same capacitance and dielectric strength would be an oversized component, which would significantly increase the costs and dimensions of the receiver.

Early electron tubes were also given an inside coating made of Aquadag in the area of ​​the actual tube system in order to prevent undesirable effects from secondary electrons knocked out of the glass bulb. Aquadag was also used in the manufacture of magic eyes . Applied between the luminous anode and the luminous layer, the discharge of electrons is facilitated and their luminosity is increased.

References and footnotes

  1. ^ A b Edward Goodrich Acheson: A Pathfinder: Discovery, Invention and Industry . How the world came to have Aquadag and Oildag; Also carborundum, artificial graphite and other valuable products of the electric furnace (=  Series of Educational Biographical Sketches of Eminent Inventors . Volume 1 ). The Press Scrap Book, New York 1910, pp. 126–128 ( online at Internet Archive [accessed March 28, 2016]).
  2. ^ Imperial Chemical to Buy Acheson Industries. The New York Times Company, March 31, 1998, accessed March 28, 2016 .
  3. ↑ According to Philips documents.
  4. ↑ Against bacterial attack.

See also

literature

  • Werner Espe: Materials science of high vacuum technology . 1: Metals and metallically conductive materials. VEB Deutscher Verlag der Wissenschaften, Berlin 1957.
  • Rudolf Goldammer: The television receiver . 3. Edition. Franzis-Verlag, Munich 1958.